This article outlines diagnostic criteria and features of the newly established diagnosis disruptive mood dysregulation disorder (DMDD), and discusses how this disorder differs from bipolar disorder in childhood. The chronic, severe, nonepisodic irritability seen in patients with DMDD contrasts with the characteristic episodic mood swing symptoms of bipolar disorder. Differentiating between the two diagnoses is important in regard to prognostic and treatment considerations, as children with DMDD are more likely to develop a mood disorder later in life rather than to develop classic bipolar disorder. Research is needed to establish clear treatment guidelines for DMDD.