Over the past decades, several psychiatric rating scales have been developed and refined, aiming to provide an objective assessment of symptom severity across distinct psychiatric disorders. These instruments allow us to assess changes in symptoms during treatments; they also serve as screening tools for diagnosis. Although most interventional studies in bipolar disorder have also focused on symptoms, recurrences, and mood states as the primary outcome variables, many researchers have highlighted the need to systematically implement cognitive and functioning measures as an essential component for planning tailored interventions. Additionally, the assessment of clinical, cognitive, and functional status proves valuable in clinical settings in order to monitor the course of the illness and the treatment response. This chapter provides an overview of the diagnostic issues on BD as well as the main psychiatric rating scales including those used to assess mood symptoms and measures to assess cognitive and psychosocial functioning.