A teratoma is a nonmalignant tumor comprised of a disorganized mixture of cells and small foci of tissue comprised of cells from all three of the embryonic germ-layers. By definition, a cell is pluripotent if it can differentiate into cells derived from all three of the embryonic germ-layers: ectoderm, mesoderm, and endoderm. In the teratoma assay, putative pluripotent stem cells (PSCs) are implanted into an immune-compromised mouse where they may proliferate and differentiate to form a teratoma. The PSCs grow at the implantation site supported by a complex mixture of factors from the local milieu, as well as circulating factors that are vital components of normal mammalian physiology. After a predetermined time of 6–12 weeks or when the tumor has reached sufficient size, it is removed and subjected to histopathological analysis. The teratoma may be further processed by immunocytochemistry and gene expression profiling. This chapter describes methods to generate teratomas through the implantation of putative PSC lines in the SCID mouse. Implantation at the following sites is described: (1) intramuscular, (2) subcutaneous, (3) under the testis capsule, and (4) under the kidney capsule.