The previous two chapters have described the generation of chimeric embryos by blastocyst microinjection and morula aggregation. This chapter describes the reimplantation of these embryos into pseudopregnant female recipient mice (foster mice) in order for the embryos to develop to term. Four different surgical techniques will be described in detail.The transfer of blastocyst-stage embryos back into the uterus is most commonly used for chimeric embryos generated by microinjection or aggregation, or for rederivation purposes when strains are transferred to a clean environment. Fertilized oocytes manipulated by pronuclear microinjection or two-cell-stage embryos are reintroduced to the infundibulum of the oviduct of pseudopregnant female mice.To generate pseudopregnant females, they need to be mated to sterile male mice. These males can be either generated by vasectomy of stud males or naturally sterile males can be used. Two different techniques for the vasectomy are shown: the vas deferens is accessed through an incision in the scrotal sac or through an abdominal incision.