This paper looks at some recent developments in the official diagnostic definitions (DSM-5) and in the research domain. The spectrum concept of mood disorders consists of the components of depression and mania, alone or in combination, on a continuum. Its international operational classification changes regularly, being based on symptoms, their duration and consequences. Causation is as yet unknown.DSM-5 excludes unipolar mania and mania with mild depression as separate diagnoses (they come under bipolar I and bipolar II disorders) and introduces a new hierarchy of manic symptoms, placing energy/activity above mood (elated, irritable). This is shown to be problematic on the basis of recent data. The validity of the duration criteria for mania (1 week), hypomania (4 days) and depression (2 weeks) is also seriously questioned. Shorter episodes are clinically very relevant. The definition of mania/hypomania is a persistent problem, contributing to frequent un- derdiagnosis of bipolar disorder in depressed patients. Other contributory factors include that patients often do not feel ill or seek treatment for the consequences of their high mood, and that hypomania can be hidden by substance use disorders (SUD). Hidden hypomanic syndromes are important because associated with treatment resistance, high comorbidity with anxiety/panic and SUD, psychotic and cognitive symptoms, dementia and higher mortality. Anxiety, too, is doubtless a mood disorder but there is still no concept which integrates anxiety with bipolar disorder and depression. Classification involves the definition of artificial subgroups and is necessary for treatment and communication but clinicians, when in doubt, (...)