Bob Rock was the principal producer of Metallica's 1991 album, also known as The Black Album.
Rock had previously worked with major rock and metal acts, including Mötley Crüe, and brought a different production approach to Metallica. The band had become dissatisfied with aspects of the sound on ...And Justice for All, especially its unusually quiet bass presence.
Recording the new album was often tense. Rock pushed the musicians to perform together more naturally and encouraged arrangements that were more direct and accessible. James Hetfield and Lars Ulrich also received production credits, so the album was a collaborative production rather than a project controlled by Rock alone.
The finished record featured a fuller, heavier, and more polished sound. Rock continued working with Metallica on later albums, including Load, Reload, and St. Anger, although the band's relationship with him changed over time.