This chapter will provide a brief overview of the various causes of dementia and their pathologies, including the most recent diagnostic criteria, and how biomarkers are beginning to be integrated into clinical practice, especially for Alzheimer disease (AD). Full clinical and pathological descriptions and formal criteria for how to diagnose the numerous known dementias should be obtained by consulting standard reference sources. The discussion here will emphasize how various pathological mechanisms in different dementias disrupt brain circuits and their neurotransmitters. We will also show how disruption of these brain circuits is linked to various symptoms of dementia, and how drugs targeting these brain circuits and their neurotransmitters lead to symptomatic improvement, emphasizing memory, psychosis, and agitation. The goal of this chapter is to acquaint the reader with ideas about the clinical and biological aspects of dementia and its current management with various approved drugs as well as novel agents on the horizon.