Protein-protein interactions are central to every cellular process like DNA replication, transcription, translation, splicing, signal transduction and cell cycle control. To elucidate the function of a gene, we need to determine the function of the gene’s encoded protein product. Identifying the role of a protein needs understanding about the proteins that interact with each other for functioning of a particular biological pathway. Assessment about function of a protein can be made by understanding protein-protein interaction studies. These inferences are based on the fact that the role of unknown proteins may be elucidated if captured through their interaction with a protein target of known function. Thus in this chapter, we attempt to summarize different in vitro methods used to identify proteins- protein interactions and to assess the strengths of these interactions.