Book excerpt: It is by no means a revelation that proteins are not uniformly distributed throughout the cell. As a result, the idea that protein molecules, because of the specificity with which they can engage in interactions with other proteins, may be aimed—via these interactions—at a restricted target, is a fundamental one in contemporary molecular life sciences. The target may be variously c- ceived as a specific molecule, a group of molecules, a structure, or a more generic type of intracellular environment. Because the concept of protein targeting is intuitive rather than expl- itly defined, it has been variously used by different groups of researchers in cell biology, biochemistry, and molecular biology. For those working in the field of intracellular signaling, an influential introduction to the topic was the seminal article by Hubbard & Cohen (TIBS [1993] 18, 172–177), which was based on the work of Cohen’s laboratory on protein phosphatases. Sub- quently, the ideas that they discussed have been further developed and extended by many workers to other key intermediaries in intracellular sign- ing, including protein kinases and a great variety of modulator and adaptor proteins.