Methods: Psychodynamic psychotherapy (PDT) and cognitive behavioural therapy (CBT) are therapies, which appear rather different in both methods and theory, but which despite these differences, in accordance to the dodo bird verdict, produce similar outcomes. This theoretical thesis examined the evidence of the efficacy of CBT and PDT in the treatment of depression, and looked on different possible factors that can explain their mechanisms of action. Results: Three randomised controlled studies were included. The quality of the studies varied in their quality as demonstrated in variations in statistical power and control group conditions. Two out of three studies indicated an equivalence of PDT to CBT for change in depressive symptoms at posttreatment. Different possible explanations for these results were discovered. One explanation was common factors, which included factors like a therapeutic alliance and insight. Another explanation was the therapies effect on an evolutionary cause of depression. The included studies indicated that less than half of the participants achieved remission. Different possible explanations for this finding were: Differences in disorder vulnerability, suboptimal cognitive capacities and lack of cultural considerations in therapy. Conclusion: Results suggest equivalence of PDT to CBT. Different possible explanations for this result were found, however further research is needed to identify the exact causal mechanisms of change in psychotherapy.