The term and concept of "comorbidity" has been mired in controversy, although there is little question that the existence of covariation among psychiatric diagnoses poses significant challenges to current models of psychiatric classification and diagnosis. The papers in this Special Section underscore a number of important issues relevant to the comorbidity between and within childhood externalizing and internalizing disorders, and illustrate both methodological and substantive reasons for such comorbidity. Weiss, Susser, and Catron's distinction among common, broad-band specific, and narrow-band specific features provides a helpful framework for understanding the comorbidity of childhood externalizing and internalizing disorders (B. Weiss, K. Susser, & T. Catron, 1998). Hierarchical models of psychopathology help to dissolve the distinction between "splitters" and "lumpers" and point to variables that may elucidate the etiology of externalizing and internalizing disorders.