Proper detection of estrus is critical in dairy husbandry. Loss of profit due to extended calving intervals, milk loss, and veterinary costs are related to incorrect detection of estrus. Despite enormous progress in the knowledge of reproductive physiology and the development of estrous detection aids, detection of estrus remains a major problem. For the detection of estrus, many factors have to be taken into account. Besides internal actions, like hormonal changes, a number of external changes (e.g., behavior) can be observed. When using visual observations, time of the day and time spent on observation have a great impact on detection rates. Today, many devices are available to aid in estrous detection, such as pedometers, mount devices, temperature, and hormone measurements. Estrous expression can be influenced by many factors, like heritability, number of days postpartum, lactation number, milk production, and health. Also, environmental factors like nutrition, season, housing, herd size, etc. play a role