The external signals that control cellular behavior are chanelled through cell surface receptors, that in turn regulate cytoplasmic signaling pathways. Work over the last ten years has indicated that the activation of these intra- cellular signaling pathways depends on a series of protein-protein and protein-phospholipid interactions mediated by modular polypeptide domains. Through their association with specific peptide motifs, these protein modules define a recognition code through which many aspects of cellular function are controlled. The articles in this book describe the varied features of these domains, and outline the structural, biochemical and genetic evidence that demonstrates their importance in co-ordinating cellular responses to external cues.