The process generally termed signal transduction involves the coordinated relay of information from extracellular cues to intracellular effectors, subsequently leading to a specified cellular response. The formation of multimeric protein complexes is a critical step in the activation of most intracellular signal transduction cascades. In many cases, these processes are initiated by a family of molecules consisting of protein association motifs known as src homology 2 and 3 (SH2 and SH3) domains. This review focuses on a group of proteins within this family that lack intrinsic enzymatic functions and consist almost entirely of SH2 and SH3 domains. Termed "adaptors," these proteins serve to physically bridge activated cell surface receptors to various intracellular signal transduction pathways. Here, I briefly summarize current knowledge concerning the various adaptor proteins and place a particular emphasis on Nck. Various data are discussed which collectively support a role for Nck in the regulation of multiple intracellular signaling events.