Multicellular organisms are all developed from single fertilized cells. Previous reports have generated a complex but incomplete picture of the developmental process from fertilization of reproductive cells at early stage to the more complicated later stage involving cell and tissue differentiation. Recent technology advances in cloning and stem cells not only provide new tools for manipulating the developmental process, but also increase the complexity of development, such as reprogramming. The research works published in this issue aim to deliver new clues for better understanding and manipulating the developmental complexity.