In the past decade, interest in and research on pediatric bipolar disorder (BD) has increased substantially. Prevalence rates of the disorder have doubled in outpatient settings, while twice as many research articles on pediatric BD were published in the past five years as in the prior decade. This review focuses on recent developments in the study of pediatric BD. We examine current research on the diagnostic boundaries of BD in youths, in particular the issues of episodicity and irritability, and provide assessment guidelines. We review data elucidating the pathophysiology of pediatric BD, with a focus on how these results may inform diagnosis. Finally, we discuss treatment approaches for pediatric BD, particularly psychotherapeutic interventions. Throughout the review, we pay particular attention to youths with severe chronic irritability, hyperarousal, and hyperreactivity, who reflect the population in whom the diagnosis of BD is most debated.