The aim of this paper is to review the concept of comorbidity as it pertains to eating disorders. The historical framing of eating disorders within medicine and psychiatry is described and then we move to what is understood by comorbidity in the current context of diagnostic compendiums. The issue of comorbidity highlights the unsatisfactory nature of the current description of illness phenotypes. There is a move to look for broader and more specific concepts such as that of endophenotypes, for example, in relationship to neuropsychology, and the response to reward and emotion. Finally, we consider how this approach may map onto treatment. For example, it may be necessary to have specific modules tailored to the relevant moderating and mediating factors.