Background: Reliable and valid diagnostics and treatment of bipolar disorders and aﬀective episodes are subject to extensive, especially methodological limitations in the clinical practice. Objective: The use of smartphones and mobile sensor technology for improvement in diagnostics and treatment of bipolar disorders. Methods: Critical discussion of current research on the use of ambulatory monitoring and digital phenotyping with bipolar disorders. Results: In many studies the observation periods were too short and the sensors applied were too inaccurate to enable reliable and valid detection of behavioral changes in the context of aﬀective episodes. Conclusion: The clariﬁcation and operationalization of psychopathological constructs to allow for the measurement of objectively observable and ascertainable behavioral changes during depressive and (hypo)manic states are essential for the successful application of modern mobile technologies in the diagnostics and treatment of bipolar disorders.