The study of protein--protein interactions is central to understanding the chemical machinery that makes up the living cell. Until recently, facile methods to study these processes in intact, living cells have not existed. Furthermore, the assignment of function to novel proteins relies on demonstrating interactions of these proteins with proteins of known function. This review describes an experimental strategy, devised to study protein--protein interactions in any intact living cells based on protein-fragment complementation assays. Applications to quantitative analysis of interactions, allosteric processes and cDNA library screening are discussed. Recently, the feasibility of employing this strategy in genome-wide biochemical pathway mapping efforts has been demonstrated.