Pregnancy begins with fertilization of the ovulated oocyte by the sperm. After fertilization, the egg undergoes time-dependent mitotic division while trying to reach the blastocyst stage and the uterus for implantation. Uterine preparation for implantation is regulated by coordinated secretions and functions of ovarian sex steroids. The first sign of contact between the blastocyst and the uterus can be detected experimentally by an intravenous blue dye injection as early as the end of day 4 or the beginning of day 5 of pregnancy. This blastocyst-uterine attachment reaction leads to stromal decidual reaction only at sites of implantation. The process of implantation can be postponed and reinstated experimentally by manipulating ovarian estrogen secretion. Stromal decidualization can also be induced experimentally in the hormonally prepared uterus in response to stimuli other than the embryo. Fundamental biological questions surrounding these essential features of early pregnancy can be addressed through the application of various techniques and manipulation of this period of early pregnancy. This chapter describes the routine laboratory methodologies to study the events of early pregnancy, with special emphasis on the implantation process in mice.