This thesis has explored the area of phenomenology (and in particular, existential phenomenology) and the application of the - phenomenological method to psychotherapy. Such exploration has been undertaken largely by considering the contributions of individual philosophers and psychotherapists.The philosophers whose contributions have been considered are Descartes, Husserl, Heidegger, and Sartre. The development from Descartes' method of universal doubt to Husserl's pure phenomenology, and from Husserl's phenomenology to the application of the phenomenological method toward studying Being (Heidegger) and toward studying man's existence (Sartre) is observed.The application of the phenomenological method to psychotherapy is investigated by considering the contributions of the existential phenorenological psychotherapists Binswanger, May, and Laing.