Depression affects 15% of Canadians resulting in serious impact on health, ability to function including social, family and work related activities. Despite the several treatment options available for managing depression including medications, many patients do not respond to treatment and experience troublesome side effects. Psychotherapies are important in the treatment of depression and several options are currently being offered at the Mood Disorders Program (MDP), St. Joseph's Healthcare Hamilton. However a simple and reportedly effective therapy called Behavioural Activation (BA) is not currently available and existing evidence supporting its' effectiveness is limited to individual therapy of community based patients who are unlike the patients seen at the MDP who may have a more severe illness. The investigators are therefore planning to study the effectiveness of BA in patients with depression as an add on therapy to existing usual care compared to wait-list added to usual care. The study outcome is reduction in depressive symptoms and improvement in quality of life. The duration of therapy is 18 weeks and all adults with depression are eligible to participate.

This study is a pragmatic randomized controlled trial to test the effectiveness of behavioural activation in depression. The pragmatic randomized controlled trial study design is a parallel 1:1 allocation comparing behavioural activation plus treatment as usual to waitlist control group plus treatment as usual. For this study the investigators will adopt the following principals simulating naturalistic real life clinical setting to test the study question based on the pragmatic design:

1. No restrictive inclusion criteria will be used. Adults with major depressive disorder will be asked to participate in this study
2. Clinicians will deliver the BA program to participants randomized to receive the intervention
3. The intervention will be an add-on to treatment as usual
4. The comparison group will receive treatment as usual that may include medications, CBT and other therapies as required and decided by their clinical care
5. The primary outcome is clinically relevant (improvement in depressive symptoms and quality of life)
6. Both the intervention and control groups will be assessed as intention to treat analysis with no measures to improve adherence to the study intervention or the comparator.

Patients with a diagnosis of depressive disorders attending the mood disorders clinic, referred for assessment of depression at the clinic or referred from community or other hospital services to the mood disorders clinic will be approached for participation in the study. In addition family practices in the community will be informed of the study and asked to refer directly to the trial. Following initial screening for eligibility, potential participants will be asked to provide written informed consent prior to any study related procedures.

The investigators will employ a parallel group design to evaluate the effects of behavioural activation intervention to improve depression related outcomes. Eligible and consenting patients will be randomly allocated to the intervention or control arms using a 1:1 allocation ratio. Allocations will be generated using the computerized system. The randomization will use a block randomization system of block sizes of 2, 4 and 6. The assignment of participants to the intervention or control arms will be done after the screening visit and enrolment of at least 20 participants to ensure balanced groups. The allocation will be done by a research personnel who is not a clinician and will not know the participant clinical status to maintain allocation concealment.

This trial is an open label trial as blinding is not possible for participants (behavioural activation intervention) or the clinician administering the intervention.

Inclusion Criteria:

• Clinical diagnosis of Major Depressive Disorder
• Must be able to provide written informed consent
• Must be able to attend program sessions

Exclusion Criteria:

• Inability to understand written and spoken English
• Primary diagnosis other than Major Depressive Disorder