and limited sustained recovery. Underpinning this efficacy gap is limited understanding of how complex psychological interventions for depression work Recent reviews have argued that the active ingredients of therapy need to be identified so that therapy can be made brief more potent, and to improve scalability. This in turn requires the use of rigorous study designs that test the present or absence of individual therapeutic elements, rather than standard comparative randomised controlled trials. One such approach is the Multiphase Optimization Strategy, which uses efficient experimentation such as factorial design to identify active factors in complex interventions. This approach has been successfully applied to behavioural he but not yet to mental health interventions. Methods/Design: A Phase III randomised, single-blind balanced fractional factorial trial, based in England conducted on the internet, randomized at the level of the patient, will investigate the active ingredients internet cognitive behavioural therapy(CBD for depression. Adults with depression(operationalized as recruited directly from the internet and from an National Health Service lmproving Access Psychological Therapies service, will be randomized across seven experimental factors, each reflecting presence versus absence of specific treatment components(activity scheduling, functional analysis, relaxation, training, absorption, self-compassion training) using a 32-condition balanced fractional factorial design(2N2. primary outcome is symptom of depression(PHQ-9) at 12 weeks. Secondary outcomes include symptoms of anxiety and process measures related to hypothesized mechanisms. Continued on next page)