Psychotherapy represents a diverse and controversial field. It is characterised by an excessive proliferation of various psychotherapeutic approaches accompanied by the sectarian attitudes of a majority of psychotherapists. In response to these, the psychotherapy integration movement was established. Within this movement three ways of psychotherapy integration have emerged, namely, theoretical integration, common factors approach and technical eclecticism. Methodological issues of theoretical integration are the focus of interest in this thesis. The current methodological recommendations in this area seem to be very limited. A specific method of assimilative integration has been proposed and the necessity of the existence of metatheoretical congruence between theories to be integrated has been emphasised. Both of these recommendations are in need of further elaboration and extension. In order to clarify some of these methodological issues, the current "state of the art" of theoretical integration is explored by comparatively analysing existing integrative theories. In this way, their similarities and differences are revealed with the unveiling of some aspects of the integrative assimilation that was used in their creation. On the basis of these findings some guidelines for future theoretical integration are proposed that might prompt further theoretical and empirical research in this area.