The delivery of a single, healthy child is the desired outcome of human assisted reproduction techniques. To attain this goal, there is an increasing movement towards single embryo transfer. Single embryo transfer also carries the added advantage of reduced multiple pregnancy rate while maintaining an acceptable overall pregnancy rate. The question is, therefore, at what stage to transfer the human embryo back to the uterus? The answer lies in selecting the embryo with the greatest potential for implantation and thus, survival. As the topic of preferential transfers on day 2, 3 or 5 remains controversial,this book researches in depth the various factors affecting embryo growth and transfer during its many stages shedding light on why blastocyst (day 5) transfer appears to be the the most optimistic choice.