Bipolar disorder (BD) occurring in children and adolescents may have a different clinical course than that diagnosed in adults, while there are no different criteria in the diagnostic classifications used. Among patients under 18 years of age, irritability, atypical course of depressive episodes, mixed episodes or very rapid phase change are much more frequently observed. Diagnostic difficulties are also overlapped by those resulting from the need to differentiate BD from other disorders which have their onset in childhood and adolescence; the probability of their coexistence should be considered concomitantly. This applies primarily to hyperkinetic disorders, behavioral disorders, borderline personality disorder or addiction to psychoactive substances. About 50% of patients present with symptoms of BD before the age of 18, and it takes on average 5 to 10 years from symptom onset to receive a correct diagnosis. Undiagnosed BD can have a negative impact on the development of emotional, social and cognitive competences, significantly affecting a patient’s functioning into adulthood. That is why it is so important for specialists working with children and adolescents to be aware of the differences in the clinical course of BD and to make the right diagnosis, taking into account the biological, developmental and systemic context.