With each successive revision of the DSM and ICD, psychiatric comorbidity has become more prevalent. The 'atheoretical' approaches of the DSM and ICD explicitly encourage multiple diagnoses with few exclusionary hierarchies, in the hope that all clinically relevant information will be captured. However, the current strategy of diagnosing 'maximal' comorbidity may not reflect 'optimal' comorbidity. Many clinicians and health information systems, particularly those in developing countries, have a limited capacity for capturing this diagnostic information, and fail to characterize additional diagnoses that are present. This article will address the evolution of our current diagnostic system as a way of understanding the emergence of comorbid psychiatric diagnoses. Alternative diagnostic approaches (a dimensional system, diagnostic hierarchies, and mixed diagnostic categories) that could be used to address the emergence of comorbid psychiatric diagnoses are considered. Future challenges for the next evolution of DSM and ICD are presented.