Protein phosphatase-1 (PP1) is a major phosphoserine/phosphothreonine phosphatase that regulates multiple physiological events in all eukaryotic cells. Action of PP1 in cells is dictated by the association of PP1 catalytic subunit with one or more regulatory subunits that define both its catalytic function and subcellular localization. This chapter describes key methods used to identify PP1-binding proteins and assess their ability to modulate PP1 functions in mammalian cells. These methods include affinity isolation of cellular PP1 complexes, analysis of direct PP1 binding, modulation of PP (protein phosphatase) activity, and testing for the presence of the newly identified PP1 complex in cells and cellular compartments. Together these techniques set the foundation for further studies that can establish the physiological significance of this PP1 complex.