This chapter presents Hermeneutic Single Case Efficacy Design (HSCED) as a method suitable for making initial claims of causal status for new therapies, or the application of existing therapies to new client populations. For this reason they wanted a client being treated in a nonstandard therapy, in order to illustrate the use of the method for treatment development research. The authors thus selected a client being seen for a panic/phobia in emotion-focused therapy. At the time this research was carried out, there was no research on the application of an experiential therapy with severe anxiety disorders other than PTSD. A key element in HSCED is the administration of a weekly measure of the client's main therapy-related problems or goals, given twice prior to the beginning of therapy, at the beginning of each session, and at later assessment points.