The endometrium is a complex tissue that is constantly influenced by ovarian steroid hormones such as estrogen and progesterone during the reproductive period. It undergoes regular cyclical changes in each menstrual cycle, with the exception of early menarche and perimenopausal period when ovulation can be irregular. This chapter provides detailed histomorphologic descriptions of the three phases of the menstrual cycle, namely proliferative, secretory, and menstrual, as well as a discussion on the dating of the endometrium. These changes are observed in the functional layer, sparing the basal layer of the endometrium. This is followed by a discussion about the different techniques for obtaining an endometrial sample, adequacy of tissue, and artifacts that may be encountered. The determination of sample adequacy can be challenging as calling minimal tissue adequate may result in false negative and on the contrary casual calling of inadequate sample may lead to an unnecessary repeat procedure. In menstrual cycle disorders, depending on whether there is continuous estrogen stimulation or estrogen deficiency, the endometrium demonstrates prolonged proliferative activity or atrophic changes, respectively. Anovulatory cycle is normally perceived as continuous proliferative activity even in the secretory phase. However, one should interpret this with caution as the duration of the proliferative phase differs in women. The current knowledge of the molecular alterations and microRNA expression in the endometrium is also described in this chapter.