The "Master" of puppets is a reference to drugs. Throughout the song the Master controls you and your life. This is evident in lyrics like, "chop your breakfast on a mirror," "the Master Of Puppets is pulling your strings, twisting your mind and smashing your dreams." Drugs are the Master while the drug user is the puppet.

This is one of the most famous and enduring songs of 1986, but it wasn't released as a single, except in France. Metal was still seen as a niche, and few radio stations would touch it. Putting it out as a single would have been pointless because it wouldn't have gotten airplay and Metallica's fans were far more likely to buy the whole album (and with a running time of 8:36, it wouldn't have fit on one side of a 45). But the album went Gold almost immediately (500,000 copies in America), and two years later went Platinum (one million), which changed perceptions. Rock radio found room for Metallica amongst offerings from Guns N' Roses and Mötley Crüe. MTV even got on board, launching Headbangers Ball in 1987. When Metallica released their next album, ...And Justice for All (1988), they were commercial enough for a single and video, which they delivered with "One." That song got them on the Hot 100 for the first time, landing at #35.

This is Metallica's most popular live song, played in concert more than any other. The crowd dutifully shouts back "Master!" at appropriate times, which could be seen as the kind of conformity the song warns against, but is simply an irresistible, adrenaline-fueled release.