Mood disorders including bipolar disorder and depression are common disabling disorders with depression affecting 11.2 to 16.0% of the general population and the lifetime prevalence of bipolar disorders at an estimated 4.4%. Although treatment with antidepressants medications is common and effective in some patients, 42.7% of patients show inadequate response to treatment with antidepressants and a large proportion (55.3%) continue to have ongoing depressive symptoms. Psychological and behavioural interventions such as cognitive behavioural therapy (CBT) and behavioural activation (BA) are effective treatment for depression alone or in combination with antidepressants. Depression can also occur in the context of bipolar disorder which is characterized by recurrent episodes of depression and mania (DSM-5). The depressive episodes within bipolar disorder may be similar to depressive disorder, however the management of these episodes is fraught with the challenge that antidepressant pharmacotherapy may precipitate manic episodes and lead to further destabilization of bipolar disorder. Therefore, an alternative to antidepressants and additional therapies are needed to support patients' recovery and mood stability, as well as to achieve better treatment response and remission. BA is not currently available in a structured format and has not been tested for its effectiveness in bipolar disorders in a specialized hospital-based program. The evidence for BA has been investigated in depression however the evidence for bipolar disorder is lacking, therefore this study aims to assess the effectiveness of BA as treatment for bipolar disorder.