Increasing efforts to identify alternate expressions of mental disorders that are broader than the DSM or ICD diagnostic criteria needed to diagnose them reflects a growing consensus that multidimensional expressions of psychiatric disorders may advance the search for underlying etiological or modulatory factors. These alternate phenotypes or “endophenotypes” (e.g., social, psychophysiological or neuropsychological abnormalities) of disorders may be more specific and amenable to objective measurement than clinical symptoms, which presumably reflects variation among smaller numbers of genes than more distal clinical symptoms. These features support the current usefulness of endophenotypes in genetic studies, and their potential usefulness in the development of strategies for early intervention. In this chapter, we review potential neurological and neuropsychological endophenotypes for schizophrenia and for schizophrenia spectrum disorders, with an emphasis on key conceptual criteria for assessing endophenotypes, including their relationships to schizophrenia, to non-psychotic relatives, and to heritability. Future directions for establishing the validity of endophenotype research are also discussed.