**1. Introduction**

From the prehistoric ages, the animals have been an integrated part of human life. With the progression of time, the dependency on domestic animals has only increased. At the present time also the human civilization cannot be imagined without the animal products we are using every day. Currently, an enormous livestock population throughout the world is contributing in achieving the global food security. But, the rapid explosion of human population which is projected to be nearly 9.7 billion within 2050 has increased the demand for animal products. So, researches are being targeted toward getting more production from the animals. The major barriers toward this goal are the different diseases and scarcity of animal feed especially in undeveloped or developing countries. Among the several diseases, problems associated with animal reproduction have always been a matter of great concern for the animal producers. The female animals having a reproductive problem cannot be conceived and produce offspring. Besides, most importantly, it also shuts the door for milk production. The major reproductive issues include

anestrus, repeat breeding, delayed estrus, different infections, etc. Among them the problem of anestrus has an incidence rate of 2.13–67.11% in the bovine population of a country like India which is the largest producer of milk [1]. Anestrus is a condition when there is absence of regular reproductive cyclicity in the female animal. Consequently, the animal becomes unproductive and causes a huge economical loss to the farmers or producers. So, it is very much important to address this problem with much care. Already, there are several established as well as developing methodologies which can induce estrus in anestrus animals. It encompasses practices like administration of hormone to the use of biostimulation. Besides inducing estrus, some technologies can also synchronize it according to the need. Synchronization of estrus can help in simplifying managemental practices as well as in some advanced technologies like embryo transfer, in vitro fertilization, cloning, etc. In this chapter we are going to discuss about the normal estrus cycle, the anestrus problem, and the methodologies developed by the researchers for induction and synchronization of estrus.
