An enigmatic yet fundamental principle of signal transduction is that parallel signaling pathways assembled from a common repertoire of enzymes are able to propagate diverse physiological responses. A key feature of such a mechanism is that separate signaling pathways are organized into localized transduction units, each tailored to respond optimally to a particular signal. Protein-protein interactions maintained by anchoring, adapter and scaffolding proteins provide the molecular glue that holds these signal transduction units together. A major objective of the signaling community is to ascertain how signals flow through compartmentalized transduction units that contain transmembrane receptors, protein kinases, phosphatases and their substrates.