The scientific discipline of endocrinology has been invaluable to our understanding of the estrous cycle. In the second half of the twentieth century the development of immunoassay technologies provided a rapid and sensitive method to quantify circulating concentrations of reproductive hormones and relate them to stage of the estrous cycle and physiological status of the animal. Ovarian ultrasonography provided the ability to track the growth and regression of ovarian structures within the same animal across the estrous cycle in real time and, in combination with hormonal profiling, accurately identify mechanisms regulating the estrous cycle and early pregnancy. Before this, the best technique had been serial collections with each animal being a single endpoint. The availability of continuous data such as daily hormone concentrations and daily follicular measurements within animals led to the improvement of methods to synchronize estrus in each of the species. Unfortunately, the use of radio-immunoassays has been declining for two decades. While enzyme-linked immunosorbent assays have been developed for many endocrine, paracrine, and autocrine factors, their primary market is human medicine and rodent models of human health, leaving those available for livestock species economically infeasible. Automated sensors such as accelerometers apply the knowledge attained through decades of endocrinology and ultrasonography studies to identify females in estrus and measure parameters of the estrous cycle that are related to fertility. The ability of automated sensors to centralize and assimilate large amounts of behavioral and physiological data from numerous animals will enhance targeted reproductive management in livestock production systems.