Mouse embryonic stem cells (mESCs) were first derived and cultured almost 30 years ago and ever since have been valuable tools for creating knockout mice and for studying early mammalian development. More recently (1998), human embryonic stem cells (hESCs) have been derived from blastocysts, and numerous methods have evolved to culture hESCs in vitro in both complex and defined media. hESCs are especially important at this time as they could potentially be used to treat degenerative diseases and to access the toxicity of new drugs and environmental chemicals. For both human and mouse ESCs, fibroblast feeder layers are often used at some phase in the culturing protocol. The feeders - often mouse embryonic fibroblasts (mEFs) - provide a substrate that increases plating efficiency, helps maintain pluripotency, and facilitates survival and growth of the stem cells. Various protocols for culturing embryonic stem cells from both species are available with newer trends moving toward feeder-free and serum-free culture. The purpose of this chapter is to provide basic protocol information on the isolation of mouse embryonic fibroblasts and establishment of feeder layers, the culture of mESCs on both mEFs and on gelatin in serum-containing medium, and the culture of hESCs in defined media on both mEFs (hESC culture medium) and Matrigel (mTeSR). These basic protocols are intended for researchers wanting to develop stem cell research in their labs. These protocols have been tested in our laboratory and work well. They can be modified and adapted for any relevant user's particular purpose.