Bipolar disorder (BD) affects between 1-3% of the world's population. People with BD experience episodes of mania or hypomania and in most cases, they experience periods of depression which can cause difficulties in daily life. Psychological therapies for people experiencing depression without mania or hypomania are widely available, but there is little research into how effective these therapies are for people with BD. Behavioral activation therapy (BA) is based on behavioral theory and has been proven to be an effective treatment for unipolar depression. It helps people re-establish healthier activity patterns and sleep regulation, especially in BD for mood stabilization. BA is theoretically and clinically well matched to the treatment of bipolar depression, but there is still very little research into offering BA to people with BD. The first aim of the current research is to implement BA for people with depression in Bipolar Disorder and study if it is feasible for this patient group. The second aim is to do a pilot study on the effectiveness of the treatment for this patient group. The research will be implemented with people seeking treatment at the specialized service for bipolar disorder at Landspítali University Hospital in Iceland. The participants will receive treatment as usual and the BA will be adjunctive. At least ten people, that are currently experiencing Bipolar Depression and are willing to take part, will receive up to 20 individual therapy sessions of BA that have been adapted for Bipolar Depression (BA-BD), and will complete regular questionnaires and interviews. The study will be a replication study to validate the previous study's findings by Kim, W. et al., 2022 in another setting.