This chapter discusses how the overlap and distinction of the primary psychotic and primary affective disorders may be understood in the context of cognition. The pattern of cognitive impairments in schizophrenia and bipolar disorders can be viewed from four dimensions: prevalence, breadth, magnitude, and course. Social cognition allows people to understand and interact with one another effectively; its impairment in both schizophrenia and bipolar disorders appears to be partially responsible for impairments in everyday functioning. Cognitive functions can be indexed at a level more proximal to neurological function through various psychophysiological methods, such as eye tracking and electroencephalogram experiments. Most comparative studies of cognition in schizophrenia and bipolar disorders focus the distinction at the level of diagnosis. Individuals with bipolar disorder and current psychotic features have cognitive impairments equivalent in magnitude to those observed in individuals with schizophrenia.