The micromanipulations of embryos and gametes are important tasks in animal biotechnology. These techniques can be used in animal cloning and transgenesis. For achieving these goals, embryonic stem cells are of principial interest. In vitro cultures of ESC can be derived from whole embryos or isolated inner cell masses (ICM). The attraction of ESC to animal biotechnological research is their pluripotency. ESC are able to contribute to all tissues, including germ lines. To obtain transgenic animals, ESC can be genetically transformed prior to the introduction to embryos. Present status of knowledge of ESC and their possible usefulness for genetic improvement of farm animals are discussed.