This study is designed to evaluate repetitive transcranial magnetic stimulation (rTMS) as a potential treatment for depression. In rTMS, a rapidly changing magnetic field passes through your scalp and skull and generates a small electrical pulses in your brain. rTMS at lower intensities has helped some people with depression but we do not know what the results will be in your case using higher intensities, or whether you will be randomized to 3 weeks of high frequency (20 cycles er second), low frequency (1 cycle per second), or inactive (sham)rTMS. You will be assigned to receive one of these types of rTMS over the left front art of your brain five times per week for the three weeks. Each rTMS treatment session should take between 20-30 minutes of actual stimulation, but weekly ratings, memory testing, and blood sampling may require several hours per week. We will also ask you to have brain imaging procedures to see if these will predict response to high vs. low frequency rTMS. If you are randomized to the 3 weeks of sham rTMS, you will have the opportunity to receive one of the active stimulation frequencies for an additional 3 weeks. Responders to any phase will be offered an additional month of rTMS prior to study termination and recommendations of alternative treatments.