From the mid-1990s through the present, studies have demonstrated a significant rise in the numbers of children and adolescents diagnosed with bipolar disorder (BD). Why is this? The present manuscript reviews several possibilities, most notably ambiguity in the diagnostic criteria for mania and how they may apply to children with functionally-impairing irritability. Furthermore, we discuss ongoing phenomenological and affective neuroscience research approaches to address those children most on the fringes of our current psychiatric nosology. In summary, these studies suggest that BD youths may be distinguished on some measures from those with chronic irritability and severe mood dysregulation, although the two groups also have some shared deficits.