The primary objectives of this clinical investigation are to (1) determine the feasibility of joining psilocybin treatment with CBT (cognitive-behavioral therapy) for patients with depression, (2) optimize CBT to most effectively integrate the psilocybin experience with psychotherapy and (3) determine the initial efficacy of psilocybin as an adjunct to cognitive-behavioral therapy (CBT) for major depressive disorder. Psilocybin will be administered orally in two doses during the course of 12 sessions of CBT to eligible study participants - a 10mg dose following the third session and a 25mg dose following the sixth session. Participants will be in active treatment for the first 4 months (psilocybin + CBT) of the study and then followed for an additional 3-months following the termination of CBT.