Glutathione-S-transferase (GST)-fusion proteins have become an effective reagent to use in the study of protein-protein interactions. They can be produced in bacterial and mammalian cells in large quantities and purified rapidly. Given that GST can be coupled to a glutathione matrix permits its use as an effective affinity column to study interactions in vitro or to purify protein complexes in cells expressing the GST-fusion. Here we provide a technical description on the utilization of GST-fusion proteins as both a tool to studying protein-protein interactions and also as a means to purify interacting proteins.