An enhancement to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-V) is currently under consideration, one that would enhance both the reliability and validity of the Diagnostic and Statistical Manual (DSM) diagnoses: the addition of a dimensional adjunct to each of the traditional categorical diagnoses of the DSM. We first review the history and context of this proposal and define the concepts on which this dimensional proposal is based. The advantages of dimensional measures over categorical measures have long been known, but we here illustrate what is known with a theoretical and a practical demonstration of the potential effects of this addition. Possible objections to the proposal are discussed, concluding with some general criteria for implementing this proposal. Copyright © 2007 John Wiley & Sons, Ltd.