Our prior work (Cook et al., 2001, 2002, 2005; Leuchter et al. 2002) has studied a new physiologic biomarker of response to SSRI and mixed-action antidepressants. The EEG-based cordance biomarker can detect the physiologic effects of successful antidepressant treatment at 48 hours, 1 week, and 2 weeks of treatment; in contrast, symptom differences between responders and non-responders did not separate until 4 weeks of treatment in our placebo-controlled trials. Additionally, the magnitude of early physiologic change was associated with the completeness of clinical response. Our biomarker has been independently studied and our findings replicated (Kopecek et al., 2006; Bareš et al., 2007, 2008). The cordance biomarker can be considered as a leading indicator or predictor of treatment outcome. As a non-invasive probe of brain physiology, it may detect early neurophysiologic changes associated with accelerated clinical response from TMS.