The recent suggestion that the classical structure-function paradigm should be extended to proteins and protein domains whose native and functional state is intrinsically unstructured has received a great deal of support. There is ample evidence that the unstructured state, common to all living organisms, is essential for basic cellular functions; thus it deserves to be recognized as a separate functional and structural category within the protein kingdom. In this review, recent findings are surveyed to illustrate that this novel but rapidly advancing field has reached a point where these proteins can be comprehensively classified on the basis of structure and function.