Adjustment disorder was introduced into the psychiatric classification systems almost 30 years ago, although the concept was recognized for many years before that. In DSM-IV, six subtypes are described based on the predominant symptoms, but no further diagnostic criteria are offered to assist the clinician. These are common conditions, especially in primary care and in consultation liaison psychiatry, where the prevalence ranges from 11% to 18% and from 10% to 35%, respectively. Yet they are under-researched, possibly due to the failure of some of the common diagnostic tools to allow for the diagnosis of adjustment disorder. Among the tools that incorporate adjustment disorder, the concordance between the clinical and interview diagnosis is very poor, with the diagnosis being made more commonly in clinical practice than the diagnostic tools allow for. Adjustment disorder is found in all cultures and in all age groups. The presence of a causal stressor is essential before a diagnosis of adjustment disorder can be made, while the symptoms vary and include those that are found in other common psychiatric disorders. It is also important to distinguish adjustment disorder from normal reactions to stressful events. Adjustment disorders are difficult to distinguish from normal responses to life's stressors, while the distinction from major depression also poses a classificatory conundrum since both are conceptually different. Adjustment disorder is a diagnosis based on the longitudinal course of symptoms in the context of a stressor, while a diagnosis of major depression is a cross-sectional one based on symptom numbers. Treatments consist mainly of brief interventions, while pharmacotherapy is limited to the symptomatic management of anxiety or insomnia. There are no robust studies demonstrating benefits from antidepressants. However, the number of studies of either type of intervention is very limited.