Protein phosphatase 1 (PP1) is a major Ser/Thr phosphatase that participates in a wide variety of cellular processes, including control of glycogen metabolism, centrosome separation, and p53/Rb-mediated growth suppression. Recent work has uncovered >50 different PP1 regulatory subunits, which allow independent regulation of PP1 in a diverse range of processes. Patricia Cohen reviews our understanding of the modes of action of these subunits. Most of these target the PP1 catalytic subunit (PP1c) to specific subcellular locations through interactions between a conserved RVxF motif in the regulatory subunit and a hydrophobic groove near the C-terminus of PP1c; the subunits GM and M110, for example, target PP1c to glycogen and muscle, respectively. Cohen also examines how signals that regulate distinct functions of PP1 are able to modulate the expression levels, conformation and phosphorylation status of PP1c regulatory subunits. She proposes that a greater understanding of these mechanisms could allow development of therapies for conditions such as diabetes, in which changes in activity of PP1 towards particular substrates would be beneficial.