Yasiin Bey (/jæˈsiːn ˈbeɪ/; born Dante Terrell Smith), best known by his stage name Mos Def (/ˌmoʊs ˈdɛf/), is an American rapper, actor, and activist. Best known for his music, Mos Def began his hip hop career in 1994, alongside his siblings in the short-lived rap group Urban Thermo Dynamics (UTD), after which he appeared on albums by Da Bush Babees and De La Soul. He subsequently formed the duo Black Star, alongside fellow Brooklyn-based rapper Talib Kweli, and they released their eponymous debut album in 1998. He was featured on the roster of Rawkus Records and in 1999 released his solo debut, Black on Both Sides. His debut was followed by The New Danger (2004), True Magic (2006) and The Ecstatic (2009). The editors at About.com listed him as the 14th greatest emcee of all time on their "50 greatest MC's of our time" list.

Prior to his career in music, Mos Def entered public life as a child actor, having played roles in television films, sitcoms, and theater. Since the early 2000s, Mos Def has appeared in films such as Something the Lord Made, Next Day Air, The Hitchhiker's Guide to the Galaxy, 16 Blocks, Be Kind Rewind, The Italian Job, The Woodsman, Bamboozled and Brown Sugar, as well as for his portrayal of Brother Sam in the Showtime drama series Dexter. He is also known as the host of Def Poetry Jam, which aired on HBO between 2002 and 2007.

Mos Def has been vocal on several social and political causes, including police brutality, the idea of American exceptionalism, and the state of African-Americans.

Mos Def attended middle school at Philippa Schuyler Middle School 383 in Bushwick, Brooklyn where he picked up his love for acting. After returning from filming You Take the Kids in Los Angeles, and getting into a relationship with an older girl, Mos Def dropped out of high school during sophomore year. Growing up in New York City during the crack epidemic of the 1980s and early 1990s, he has spoken about witnessing widespread instances of gang violence, theft and poverty in society, which he largely avoided by working on plays, Off-Off-Broadway and arts programs. In a particularly traumatic childhood experience, Mos Def witnessed his then five-year-old younger brother Ilias Bey (b. Denard Smith) get hit by a car. Bey, who later adopted the alias DCQ, was described by Smith as "my first partner in hip hop".

Mos Def began his music career in 1994, forming the group UTD (or Urban Thermo Dynamics) along with younger brother DCQ and younger sister Ces. In 2004, they released the album Manifest Destiny, their first and only release to date. The album features a compilation of previously unreleased and re-released tracks recorded during the original UTD run.

In 1996, Mos Def emerged as a solo artist and worked with De La Soul and Da Bush Babees, before he released his own first single, "Universal Magnetic" in 1997.

Mos Def signed with Rawkus Records and formed the rap group Black Star with Talib Kweli. The duo released an album, Mos Def & Talib Kweli Are Black Star, in 1998. Mostly produced by Hi-Tek, the album featured the singles "Respiration" and "Definition", which both reached in the Billboard Hot R&B/Hip-Hop Songs chart.

Mos Def released his solo debut album Black on Both Sides in October 1999, also through Rawkus. The single "Ms. Fat Booty" charted, while the album reached #25 on the Billboard 200. Around this time he also contributed to the Scritti Politti album Anomie & Bonhomie.

In January 2002, Rawkus Records was taken over by Geffen Records, which released his second solo album The New Danger in October 2004. It included contributions by Shuggie Otis and Bernie Worrell, Doug Wimbish, and Will Calhoun as the Black Jack Johnson Band. The album reached #5 on the Billboard 200, making it the most successful for the artist to date. The single "Sex, Love & Money" charted, and was nominated for a Grammy Award. Mos Def's final solo album for Geffen Records, titled True Magic, was released in 2006.

On November 7, 2007, Mos Def performed live in San Francisco at The Mezzanine venue. The performance was recorded for an upcoming "Live in Concert" DVD. During the event, he announced that he would be releasing a new album to be called The Ecstatic. He performed a number of new tracks; in later shows, he previewed tracks produced by Madlib and was rumored to be going to Kanye West for new material. Producer and fellow Def Poet Al Be Back revealed he would be producing on the album as well. The album was released on June 9, 2009; but only Madlib's production had made the cut, along with tracks by Preservation, The Neptunes, Mr. Flash, Madlib's brother Oh No, a song by J. Dilla, and Georgia Anne Muldrow.

Mos Def appears alongside Kanye West on the track "Two Words" from The College Dropout album, the track "Drunk And Hot Girls" and the bonus track "Good Night" off West's third major album, Graduation. In 2002, he released the 12" single Fine, which was featured in the Brown Sugar Motion Picture Soundtrack.

Mos Def also appears on the debut album from fellow New Yorkers Apollo Heights on a track titled, "Concern." In October, he signed a deal with Downtown Records and appeared on a remix to the song "D.A.N.C.E." by Justice. He appeared on Stephen Marley's debut album Mind Control on the song "Hey Baby." In 2009, he worked with Somali-Canadian rapper K'naan to produce the track "America" for K'naan's album Troubadour.

In April 2008, he appeared on the title track for a new album by The Roots entitled Rising Down. The new single, "Life In Marvelous Times", was made officially available through iTunes on November 4, 2008, and is available for stream on the Roots' website Okayplayer.

In April 2009, Mos Def traveled to South Africa for the first time where he performed with The Robert Glasper Experiment at the renowned Cape Town International Jazz Festival. He treated the South African audience with an encore introduced by his own rendition of John Coltrane's "A Love Supreme", followed by a sneak preview of the track "M.D. (Doctor)".

Mos Def has designed two pairs of limited edition Converse shoes. The shoes were released through Foot Locker stores on August 1, 2009 in limited amounts.

In late 2009, Mos Def created his own clothing line with the "UNDRCRWN" brand called the "Mos Def Cut & Sew Collection". The items were released in select U.S. stores and almost exclusively on the UNDRCRWN website. 2009 also found Mos Def among the MCs aligning themselves with American entrepreneur Damon Dash's DD172 and collaborating with American blues rock band the Black Keys on the Blakroc album, a project headed by the Black Keys and Damon Dash. Mos Def appeared with Harlem-bred rapper Jim Jones and the Black Keys on the Late Show with David Letterman to perform the Blakroc track "Ain't Nothing Like You (Hoochie Coo)".

In March 2010, Mos Def's song "Quiet Dog Bite Hard" was featured in Palm's "Life moves fast. Don't miss a thing." campaign.

Mos Def features on the first single, "Stylo", from the third Gorillaz album, Plastic Beach, alongside soul legend Bobby Womack. He also appears on the track titled "Sweepstakes".

In September 2010, after appearing on Kanye West's G.O.O.D. Friday track "Lord Lord Lord", Mos Def confirmed he had signed with GOOD Music. Mos Def has been an active contributor to the recovery of the oil spill in the Gulf, performing concerts and raising money towards repairing its damages. In June 2010, he recorded a cover of the classic New Orleans song originally by Smokey Johnson, "It Ain't My Fault" with the Preservation Hall Jazz Band, Lenny Kravitz and Trombone Shorty.

In September 2011, Mos Def announced that he legally changed his name to Yasiin Bey, and would continue to go by that name. Shortly after that announcement, he recorded as the narrator of the children's hip hop musical, Pacha's Pajamas: A Story Written By Nature.

In January 2012, it was reported that Mos Def and Talib Kweli had begun "to resurrect" Black Star.

In 2015, Def was featured on A$AP Rocky's second studio album At. Long. Last. ASAP, on the track "Back Home", alongside Acyde and the deceased A$AP Yams. Bey again revived his Mos Def moniker for two new songs in August 2015, titled "Basquiat Ghostwriter" and "Sensei on the Block", respectively.

On January 19, 2016, Mos Def announced his retirement from both the music and film industries on Kanye West's website: "I'm retiring from the music recording industry as it is currently assembled today, and also Hollywood, effective immediately. I'm releasing my final album this year, and that's that." After announcing his retirement, he expressed gratitude to everyone who has supported him over the years and revealed his intention to enter the fashion industry and complete a handful of films. Bey also confirmed he still planned to release a collaborative project with Ferrari Sheppard called Dec 99th.

In October 2016, a planned concert in London was cancelled due to travel restrictions imposed on the artist, while other European dates suffered the same fate. On October 14, 2016, Yasiin Bey posted a video to Facebook where he announced that he's still planning to retire: "I'm retiring for real this year, this week. With the 17th anniversary of Black on Both Sides being released, I am grateful to have had the career that I have been able to enjoy." He also announced one last concert which he will be live streaming from Cape Town, South Africa. In November 2016, he was granted the ability to leave but not reenter South Africa and was put on "South Africa's 'undesirable persons' list." He said he would perform one show in Harlem and three in Washington D.C. after leaving South Africa. He announced that Dec 99th, his final album, would be released on December 9. He released three singles from it "Local Time," "N.A.W." and "Seaside Panic Room." On December 5, 2016, he announced the title of two albums, Negus in Natural Person and As Promised, the latter of which is a collaboration with Southern hip hop producer Mannie Fresh, initially titled OMFGOD. His claims of retirement were seen as questionable as he played shows on Wednesday September 13, 2017, at the Fox Theater in Oakland as part of Black Star and performed on Gorillaz' Humanz World Tour, performing "Stylo" with Peven Everett, who filled in for Bobby Womack, who passed away in 2014. He also has yet to release or announce a release date for the last two albums of his career.

In February 2018, Mos Def announced, on stage, a new Black Star studio album to be released in 2018. The album will be produced by Madlib. No specific release date was made available. Later on that same year, Bey appeared on the titular track of Kids See Ghosts, the collaborative effort of Kanye West and Kid Cudi.

Prior to his career in music, Mos Def entered public life as a child actor, having played roles in television movies, sitcoms and theater, some of which were under the name Dante Beze. At the age of 14, he appeared in the TV movie God Bless the Child, starring Mare Winningham, which aired on ABC in 1988. He played the oldest child in the 1990 family sitcom You Take the Kids shortly before it was cancelled. In 1995 he played the character Dante, Bill Cosby's sidekick on the short-lived detective show, The Cosby Mysteries. In 1996 he also starred in a Visa check card commercial featuring Deion Sanders. In 1997 he had a small role alongside Michael Jackson in his short film and music video Ghosts (1997).

After brief appearances in Bamboozled and Monster's Ball, Mos Def played a rapper who is reluctant to sign to a major label in Brown Sugar. He was nominated for an Image Award and a Teen Choice Award.

In 2001, he took a supporting role to Beyoncé Knowles and Mehki Phifer in the MTV movie Carmen: A Hip Hopera as Lt. Miller, a crooked cop.

In 2002, he played the role of Booth in Suzan-Lori Parks' Topdog/Underdog, a Tony-nominated and Pulitzer-winning Broadway play. He and co-star Jeffrey Wright won a Special Award from the Outer Critics Circle Award for their joint performance. He played Left Ear in the 2003 film The Italian Job. That same year he appeared in the music video You Don't Know My Name of the song by Alicia Keys.

In television, Mos Def has appeared on NYPD Blue, on Comedy Central's Chappelle's Show, and has hosted the award-winning HBO spoken word show, Def Poetry since its inception. The show's sixth season aired in 2007. He also appeared on the sitcom My Wife and Kids as the disabled friend of Michael Kyle (Damon Wayans).

Mos Def won "Best Actor, Independent Movie" at the 2005 Black Reel Awards for his portrayal of Detective Sgt. Lucas in The Woodsman. For his portrayal of Vivien Thomas in HBO's film Something the Lord Made, he was nominated for an Emmy and a Golden Globe, and won the Image Award. He also played a bandleader in HBO's Lackawanna Blues. He then landed the role of Ford Prefect in the 2005 movie adaptation of The Hitchhiker's Guide to the Galaxy.

In 2006, Mos Def appeared in Dave Chappelle's Block Party alongside Black Star partner Talib Kweli, while also contributing to the film's soundtrack. He was also featured as the banjo player in the Pixie Sketch" from Chappelle's Show: The Lost Episodes, though his appearance was edited out of the DVD. He starred in the action film 16 Blocks alongside Bruce Willis and David Morse. He has a recurring guest role on Boondocks, starring as Gangstalicious. He is also set to be in Toussaint, a film about Haitian revolutionary Toussaint Louverture, opposite Don Cheadle and Wesley Snipes. He made a cameo appearance as himself in the movie Talladega Nights: The Ballad of Ricky Bobby.

In 2007, Mos Def narrated the PBS-broadcast documentary Prince Among Slaves.

In 2008, Mos Def starred in the Michel Gondry movie Be Kind Rewind, playing a video rental store employee whose best friend is played by co-star Jack Black. He also portrayed Chuck Berry in the film Cadillac Records, for which he was nominated for a Black Reel Award and an Image Award.

In 2009, he appeared in the House episode entitled "Locked In" as a patient suffering from locked-in syndrome. His performance was well-received, with E! saying that Mos Def "delivers an Emmy-worthy performance." He was also in the 2009 film Next Day Air.

In 2010, he appeared on the children's show Yo Gabba Gabba! as Super Mr. Superhero. He also appeared in A Free Man of Color, John Guare's play at the Vivian Beaumont Theatre.

In 2011, he began a multi-episode appearance on the sixth season of Showtime television series Dexter. He played Brother Sam, an ex-convict who has supposedly found religion despite finding himself in violent situations.

In January 2016, Mos Def announced his retirement from both the music and the film industry on Kanye West's website. In March 2016, it was announced that he had been attached to star in "his last live-action film", The Disconnected, a science fiction thriller dealing with policing, identity, and the intersection of technology and humanity.

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