This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.While the traditional natural-scientific model of psychology has in recent decades been extensively criticized, the implications of this criticism for the criteria of epistemologically legitimate psychological knowledge have remained unclear. It is suggested that the production of psychological knowledge should be considered in terms of communicative interaction. Two basic modes of communicative interaction - one-directional and two directional - are proposed. Epistemologically legitimate psychological knowledge requires the adoption of the latter mode. Agreement between the investigator and the person(s) whose conduct is investigated is introduced as the pivotal criterion of the validity of psychological propositions. If psychological knowledge is based on the understanding of the meaning of actions, and if meaning may only be ascertained by means of two directional communicative interaction, agreement seems to be both (a) necessary and (b) sufficient criterion of epistemological validity. Three types of counter-arguments to this view are examined: Habermas's notion of universal rationality, Grunbaum's defence of the pistemological status of the unconscious and Smedslund's common sense psychology. None of these pose any serious challenge to the proposition of agreement as criterion of validity. It is concluded that agreement between the investigator and the other participants in an investigation is the basic criterion of validity in psychology. This conclusion requires a reassessment of the notions of objectivity, relativism and intersubjectivity