"Baby" is a song by Canadian recording artist Justin Bieber. It was released as the lead single from the latter half of Bieber's debut album, My World 2.0. The track was written by Bieber with Christopher "Tricky" Stewart and Terius "The-Dream" Nash, both of whom worked with Bieber on "One Time", and also by R&B singer Christina Milian and labelmate, rapper Ludacris.

It was available for digital download on January 18, 2010. The song received airplay directly after release, officially impacting mainstream and rhythmic radio on January 26, 2010. The song is uptempo R&B, blending together dance-pop and hip-hop elements, while using influences of doo-wop music. The song has received positive reviews from critics who complimented the song's effective lyrics and chorus, and commended Ludacris' part and the song's ability to have an urban twist.

The song was a commercial success, peaking at number one in France, and charting in the top ten of the United Kingdom, Canada, the United States, Australia, Norway, Japan, Ireland, Hungary, Belgium (Flanders), Slovakia and New Zealand. The accompanying music video takes place in a mall/bowling-alley setting. The video features several cameos such as Drake, Lil Twist and Tinashe, in which Bieber chases after a girl. As of May 2013, the song has sold 3.9 million digital downloads in the United States.

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