When a somatic cell nucleus is transplanted into an egg or an oocyte, the transplanted nucleus can be reprogrammed to support early embryonic development so that the reconstructed embryo gives rise to a cloned animal. Nuclear reprogramming of somatic nuclei is induced by maternal components stored in eggs and oocytes. These endogenous reprogramming factors and mechanisms have been explored for decades in mammals and amphibia. There are several ways of investigating reprogramming mechanisms, including nuclear transfer to eggs/oocytes and incubation in egg/oocyte extracts. In this review I describe the type of reprogramming events induced in each system and what factors in eggs and oocytes are responsible for these. Based on our current knowledge, I propose a model for the early phase of nuclear reprogramming in eggs and oocytes.