This review discusses issues and controversies with respect to the construct of a mental disorder, models of etiology and pathology, and domains of psychopathology. Fundamental to the science of psychopathology is a conceptualization of mental disorder, yet inadequate attention is being given to the differentiation of normal and abnormal psychological functioning in current research. The boundaries between mental and physical disorders are equally problematic. Neurophysiological models are receiving particular emphasis in large part because of the substantial progress being made in documenting and clarifying the important role of neurophysiological structures and mechanisms in etiology and pathology. However, this attention might be at the expense of the recognition of equally valid psychological models. Problematic diagnostic boundaries are also considered, including those within and between different classes of disorder. Dimensional models may offer a more precise and comprehensive classification of psychopathology.