Transcranial magnetic stimulation (TMS) is an accepted, evidence-based treatment option for the treatment of depressive episodes in patients with major depressive disorder. However, due to the lack of TMS studies in bipolar depressed patients, the antidepressant efficacy in bipolar depression remains unclear. This study therefore aims to investigate the clinical efficacy of continuous theta burst stimulation (cTBS) on the right DLPFC as an add-on treatment in bipolar depression. Theta burst stimulation, a relatively new form of TMS, is assumed to have similar clinical effects as the more regular rTMS parameters (high frequency rTMS administered to the left dorsolateral prefrontal cortex and low frequency rTMS administered to the right dorsolateral prefrontal cortex), but it has the advantage that it requires less stimulation time to produce longer lasting effects in the human cerebral cortex. Objectives Primary objectives - To investigate the efficacy of continuous theta burst TMS as an add-on treatment in bipolar depression (type I/II). Secondary objectives - To investigate the effect of continuous theta burst TMS as an add-on treatment on cognitive and psychomotor symptoms in bipolar depression (type I/II). - To investigate the effect of continuous theta burst TMS as an add-on treatment on levels of biological markers of bipolar depression (for example neurotrophic factors or cytokine levels). - To investigate the possible predictive value of cognitive and psychomotor symptoms and biological markers for response to cTBS TMS treatment in bipolar depression (type I/II). - To investigate the safety profile of cTBS TMS in bipolar depression (type I/II). Overview of study design (cfr image 1) Flowchart of the TMS procedure. Baseline measurements will be assessed at T0. cTBS treatment is spread over four succeeding days in week 2. All participants will be randomized to receive real or sham CTBS. A second assessment (T1) will be exactly one week (+/- 1 day) after T0 and a third assessment will be exactly two weeks (+/- 1 day) after T0.