This protocol describes the production of teratomas and teratocarcinomas used to determine the in vivo developmental potential of adult or embryonic tissues, including early-stage embryos, somites, and tail bud, as well as embryonic and other pluripotent stem cells. Resulting tumors are screened by histology and markers to assess differentiation of the tissues. Methods and sites may vary, depending on cell and tissue type. Subcutaneous injections are easy to perform and do not need surgical manipulations. Suspension of cells in Matrigel Basement Membrane (BD Biosciences) promotes their anatomical localization and is a commonly used, efficient, and reproducible approach.