Co-immunoprecipitation is a common method used to determine protein-protein interactions. This method typically exploits the highly specific interaction between a monoclonal antibody and a protein of interest. Providing that the antibody-protein interaction does not interfere with the ability of the protein to interact with other proteins in a cell lysate, a protein complex containing the antibody, the protein of interest, and its interacting proteins could be isolated on protein A Sepharose beads and purified from the cell lysate. This chapter details many of the techniques and associated procedures of co-immunoprecipitation and highlights many of the concerns and considerations associated with each step of this method.