The corpus luteum (CL) is a pivotal structure in the regulation of the estrous cycle and in the establishment of pregnancy. It is one of the most vascularised organs in the body, undergoing rapid cellular and vascular changes in order to fulfill its role. B-Mode and Doppler ultrasound has enabled further research into the equine CL and can be utilised as a clinical tool in the assessment of luteal form and function. An understanding of the normal ultrasonographic variations of luteal development and regression is essential to enable clinicians to make an assessment of luteal age as well as make clinical decisions in the gynaecological management of mares under their care. Although advances in the management of brood mares have led to an increase in conception rates, early embryonic death (EED) is still a major economic concern in equine practice. Because EED is in almost all cases recognized retrospectively, it is not surprising that CL resp. progesterone insufficiency is assumed to cause EED. However, there is little scientific evidence to support this hypothesis. Despite preventive managerial measures there are few pharmacological means to improve function of the CL (e.g. Regumate®, GnRH, hCG) in the mare.