The objective of this article was to propose a novel approach, referred to as the study of trans-disease processes (TDPs), to the neuroscientific study of disease processes in general and to co-morbid diseases in particular. The features of this approach are outlined; one potential TDP-delay discounting, which may help account for the co-morbidity of cigarette smoking and schizophrenia-is explored; and the concept of TDPs is contrasted with the concept of endophenotypes. TDPs have the potential for a variety of positive impacts on science.