Depression currently affects close to 2 million Canadians and is the leading cause of disability worldwide. Pharmacological treatments (antidepressant medication) and psychological treatments such as cognitive-behavioural therapy are available for depression, but the majority of those who receive treatment have an unsatisfactory response. On average, the combination of pharmacological and psychological treatment achieves better results than either treatment alone. However, the apparently superior results of combination treatment may be due to the fact that different individuals preferentially respond to pharmacological or psychological treatment. The invesitagtors have discovered several clinical factors and biomarkers that predict poor response to commonly used antidepressant medication: history of childhood maltreatment, loss of interest and reduced activity, a biomarker of systemic inflammation, and a genetic marker of sensitivity to environment. Indirect evidence suggests that the same factors may indicate the need for psychological treatment, but their usefulness as differential predictors of psychological and pharmacological treatment outcomes remains to be established.