Although some empirical work has now been added to the larger body of case material, preschool bipolar disorder (BPD) remains a highly ambiguous diagnostic area. This is notable in the context of the significant progress that has been made in many other areas of psychopathology in the preschool period. While there is a need for well controlled empirical investigations in this area, a small but growing body of empirical literature suggests that some form of the disorder may arise as early as age 3. The need for large scale and focused studies of this issue is underscored by the high and increasing rates of prescriptions of atypical antipsychotics and other mood stabilizing agents for preschool children with presumptive clinical diagnosis of BPD or a related variant. Clarifying the nosology of preschool BPD may also be important to better understand of the developmental psychopathology of the disorder during childhood. Data elucidating this developmental trajectory could then inform the design of earlier potentially preventive interventions that may have implications for the disorder across the lifespan.