Apathy is increasingly recognised as a common behavioural syndrome in psychiatric disorders, but it is conceptually ill defined. The aim of this study was to examine the concept of apathy as it is currently used in neurology and psychiatry, by review of the literature and conceptual analysis. There is no consensus on diagnostic criteria for apathy as a syndrome. Apathy is mostly defined as a disorder of motivation, and operationalised as diminished goal oriented behaviour and cognition. There is discussion about whether an emotional dimension should form part of the definition of apathy. Abulia is considered a more severe type of apathy, but its nosological position is still unclear. A structured clinical interview and a proposal for diagnostic criteria for apathy in dementia have been recently validated. There are several valid and reliable scales to measure the severity of apathy in patients with psychiatric and neurological disorders. In summary, apathy is increasingly recognised as a common behavioural syndrome associated with neuropsychiatric disorders. There is a need for consensus on diagnostic criteria to facilitate future research. From a nosological perspective, future studies should examine the overlap with other psychiatric and neurodegenerative conditions and further validate specific diagnostic and assessment tools.