Apathy is an increasingly recognized concomitant of a broad range of central nervous system disorders. Nevertheless, its nosology, pathogenesis and therapy remain shrouded in confusion and controversy. As yet, there is little consensus regarding methods for detecting apathy, or distinguishing it from depression, or for assessing its severity. Many now regard the apathy syndrome as primarily reflecting a lack of motivation that compromises emotional, cognitive, and overt behavioral function. Even though under-recognized and under-diagnosed, apathy hardly appears uncommon: current epidemiologic studies suggest over 10 million Americans may be affected. Its reported frequency in various neurologic and psychiatric conditions varies widely, from less than 10 to over 80%, reflecting differences in population characteristics and assessment procedures. Often apathy has been associated with such neurodegenerative disorders as Alzheimer's disease, Parkinson's disease, and fronto-temporal dementia. But it also occurs in those with psychiatric disorders such as schizophrenia and major depression. Clinical, neuropathologic, and neuroimaging observations increasingly suggest that apathy reflects dysfunction of frontal-subcortical circuits, especially those linking the ventromedial prefrontal cortex to related regions in the basal ganglia. Therapeutically, numerous small studies suggest that psychostimulants, dopaminergics, and cholinesterase inhibitors may benefit those manifesting this syndrome. However, no adequately powered, randomized controlled trials have reported success and no medication have ever been approved for this disorder. The accelerating pace of current research nevertheless promises to improve our understanding of apathy and to better address the unmet medical needs of those suffering its consequences.