This chapter provides an overview of current thinking regarding the supporting theory of psychodynamic psychotherapy. Rather than going through theoretical constructs in historical order of when each theory was proposed, they are presented as a composite of past and present thinking that the authors have found to be clinically relevant. It commences by describing theories on the early development of the infant and the creation of the internal world and object relations. The chapter them moves on to the issue of accommodating to the world as it is experienced by the infant, outlining circumstances leading to adaptive and less adaptive development. The chapter then outlines ‘core theory’ which covers more traditional psychodynamic concepts such as conflict, resistance, and defence mechanisms with an emphasis on projective identification. The role of this latter defence mechanism is linked with the theoretical constructs of transference and countertransference. There is a section on the narcissistic constellation in order to help the reader negotiate later sections in the book. Finally the chapter concludes with an introduction to theories as to how change is effected in psychodynamic psychotherapy.