Many decades have passed since the first conceptualisation of a bipolar continuum. Despite fair agreement that bipolar is not a discrete construct, there remains no universally agreed model as to how best to conceptualise a bipolar spectrum, with outstanding challenges including how and where to demarcate thresholds within it. The lack of universal consensus as to what does, and does not, constitute a bipolar spectrum disorder is a clear clinical challenge. Arguments persist as to the benefits and pitfalls of broadening the current diagnostic criteria, but it remains the case that there is still an underdiagnosis of bipolar disorders and that people meeting various subthreshold criteria share many characteristics with people who have diagnosable bipolar disorders. The evidence base for treating people with cyclothymia, depression with mixed features, and other presentations in the “gap” between formal bipolar and unipolar conditions is scant and calls for future substantial, high-quality trials.