Infection by a temperate phage can lead to death of the bacterial cell, but sometimes these phages integrate into the bacterial chromosome, offering the potential for a more long-lasting relationship to be established. Here we define three major ecological and evolutionary benefits of temperate phage for bacteria: as agents of horizontal gene transfer (HGT), as sources of genetic variation for evolutionary innovation, and as weapons of bacterial competition. We suggest that a coevolutionary perspective is required to understand the roles of temperate phages in bacterial populations.