The clinical confusion surrounding childhood ADHD and bipolar disorder centers on overlaps between severe ADHD with mood lability and mania/hypomania. Perplexity has been exacerbated by the removal of mood symptoms from the diagnostic criteria for ADHD and a lack of stringent criteria for a manic/hypomanic episode. This review summarizes current knowledge of the relationship between ADHD and bipolar disorder, the rates with which ADHD and bipolar disorder coexist in youth of differing ages, their presence in community, clinical, and high risk samples, and their longitudinal course. Treatment studies are reviewed, highlighting findings in comorbid cases, which support the efficacy of stimulants and other agents for ADHD without worsening mood symptoms, and efficacy of second generation antipsychotics for bipolar disorder. In conclusion, a lack of clarity regarding the diagnostic boundaries between childhood ADHD and bipolar disorder remains, however, treatments targeting symptoms of each disorder when comorbid, provide some efficacy.