The corpus luteum is a transient endocrine gland which is essential for the maintenance of early pregnancy in mammals, however the physiological mechanisms which control luteal lifespan are poorly understood. The aim of this thesis is to investigate potential control points in luteal maintenance and regression by studies utilising human tissues from throughout the luteal phase and in simulated early pregnancy. In particular, the roles of luteal growth and angiogenesis, apoptosis and immune cells are examined. Results of these studies indicate that although angiogenesis is maximal in the early luteal phase, growth of blood vessels does not vary during luteal maintenance and regression. Similarly, although the apoptotic protoncogenes bcl-2 and bax are present in the human corpus luteum, expression of these factors remains constant as luteal function changes. Leukocytes are present throughout the luteal lifespan and local cytokine production may be important in causing the ingress of immune cells which is observed during luteal regression. Cells of steroidogenic, vascular and immune cell compartments of the human corpus luteum may interact in complex ways to bring about changes in the function and structure throughout the lifespan of the gland.