Although research on psychotherapy advanced considerably during the past years, the demand for empirical outcome and clinical studies providing evidence for the effectiveness of the applied treatment is still growing and can be considered a critical challenge for psychotherapy as such as well as for individual schools of psychotherapy, like Viktor E. Frankl's logotherapy and existential analysis which is the focus of this chapter. In order to address this demand, after giving a short overview about the history of research on logotherapy, this chapter provides a systematic review of 42 empirical studies on the theoretical foundations and the effectiveness of logotherapy covering the years 2010–2015. The implications of these findings for future research on logotherapy are discussed.