Severe, non-episodic, irritability is a prevalent (3.2%) symptom in children and adolescents which causes severe psychiatric impairment in them. There is controversy as to how they should be diagnosed and treated, because they don’t fit into any existing DSM category. Literature about diagnostic issues of severe, nonepisodic, irritability and Bipolar Disorder (BD) in youth, published over the past 25 years were reviewed and summarized. Within the last decade, it has been suggested that severe, non-episodic, irritability is a developmental phenotype of pediatric BD. This causes a marked upsurge in the rate of the diagnosis of pediatric BD in the past decade, although there is incomplete evidence regarding the validity of this school of thought in the existing literature. Therefore, over the past 8 years, researchers have compared youth with severe, non-episodic, irritability with youth who meet DSM-IV criteria for BD, in order to develop an appropriate diagnostic criterion for BD in children and adolescents. The careful weighing of both scientific and clinical factors led to adding a new diagnosis to the DSM-V Mood Disorder section: Temper Dysregulation Disorder with Dysphoria (TDD). Having a home in DSM, can not only prevent assigning the diagnosis of BD to a substantial number of youth who don’t actually meet BD criteria, but can also provide them with the intensive care and treatment they require. However, now the major, still not answered, nosological question is whether TDD is a developmental phenotype of BD, or it is a syndrome on the continuum with anxiety disorders, unipolar depression, Oppositional Defiant Disorder (ODD), and Attention Deficit Hyperactivity Disorder (ADHD). Considering TDD as a separate diagnosis can focus attention on generating effective treatments for a common and severe clinical syndrome, and will foster further research on severe, non-episodic, irritability in youth.