Signal transduction pathways are typically controlled by regulated protein-protein interactions, mediated by dedicated interaction domains. The prototype for such interactions involves the recognition of phosphotyrosine sites on receptor tyrosine kinases by the SH2 domains of cytoplasmic signaling proteins. Many other types of post-translational modifications are also recognized by specific interaction domains, which therefore provide a general mechanism to couple the dynamic state of the proteome to cellular responses. There are ~100 classes of interaction domains present in human proteins, with each class being represented by up to 300 members. Interaction domains therefore represent a prevalent feature of the proteome. I will argue that they provide a simple mechanism for the evolution of new biological functions, and conversely that aberrant protein-protein interactions are a common basis for disease. Adaptor proteins are composed exclusively of interaction sequences, and serve to couple signaling receptors to specific components of the core cellular machinery, thereby shaping the cellular response to a particular biological input. I will discuss the ability of adaptors to control cellular behaviour, particularly with relation to phosphotyrosine-mediated control of the actin cytoskeleton, and also the mechanisms by which pathogenic proteins can exploit this molecular device to re-wire cellular function.