Even in the most optimal conditions, the chance of human conception is no more than 30%–40% within each menstrual cycle. Studies have shown that of all spontaneous conceptions, only 40% are clinically evident [1]. The cumulative pregnancy rate within the first year of attempts at conception is close to 90% but plateaus after [2]. In this chapter, the physiology of implantation is reviewed, highlighting the prerequisites to implantation, factors that are thought to impede implantation, and the crucial role that ultrasound plays in identifying these factors. Particular emphasis is placed on assisted reproduction. Beyond implantation and the ongoing growth of the embryo are also covered.