Brain imaging techniques combined with genetics contribute to developing an understanding of the pathophysiological mechanism of disease and treatment response. Psychiatric genetics identifies associations between allelic variation in genes and the risk for the development of psychiatric illness. Although the genetic association of apoE4 with the development of Alzheimer's disease is well established, there is a need to devise strategies that can fill in the missing links between apoE gene variation and the pathophysiological mechanism of Alzheimer's disease. Brain imaging addresses and bridges this gap. The combination of imaging, allelic characterization, and clinical response synergistically contributes to understanding the role apoE4 plays in the development of Alzheimer's disease and its response to treatment. An analogous argument is made for combining brain imaging with allelic subtyping to understand clinical antipsychotic response in schizophrenia. This chapter establishes the efficacy of new pharmacological treatments for groups of patients with schizophrenia and Alzheimer's disease.