Apathy is characterized by loss of motivation, decreased initiative, and emotional blunting. It is highly prevalent in neurological, and psychiatric disorders like Alzheimer's disease, traumatic brain injury, schizophrenia, Parkinson's disease, Huntington's disease, cerebrovascular disorders, and mild behavioural impairment. It has negative outcomes including impairments in activities of daily living, caregiver burden, and higher rates of institutionalization andmortality. The definition of apathy has changed over the years alongside the development of diagnostic criteria and apathy scales and measurements. Apathy is emerging as a treatment target with interest in pharmacological, non-pharmacological, and neuromodulatory treatments for apathy. There is also anincreased understanding of the neurobiology of apathy with functional and structural neuroimaging research studies. This book is a comprehensive, in-depth review from experts in neurology and psychiatry. It reviews the current state of apathy in these various disorders while also summarizing apathy diagnostic criteria, scales and measurements, neuropathology, and treatments.