Decidual cells represent a morphologically distinct class of cell appearing within the endometrium of mammalian uteri. The decidual cell reaction is a complex process that occurs in the endometrium during pregnancy, involving changes in gene expression and cellular morphology. Decidualization is the transformation of endometrial stromal cells into specialized establishing implantation decidual cells, crucial for and maintaining pregnancy. This review explores the intricate mechanisms underlying the decidual cell reaction, focusing on the roles of key hormones such as estrogen, progesterone, relaxin and androgen. Additionally, it discusses the expression of critical genes and proteins like MUC1, HB-EGF, and BMP2 in regulating decidualization. The involvement of immune cells like natural killer cells and the use of animal models to study decidual development are also highlighted.