**1. Introduction**

Complex traits are typically influenced by multiple genes by their combined contributions and modifications of environmental factors. However, a few genes or loci account for most variation between individuals for any given domesticated species. Researchers develop various methods, such as marker-assisted selection (MAS), to improve production and reproduction, and performance traits because of their economic significance in dairy and beef cattle over the last 50 years. This chapter presents issues about the major traits with economic values for the genetic improvement of livestock reproduction. It also covers aspects from basic information about physiological mechanisms of ovarian follicular development in ruminants to incidence of multiple ovulations to the fundamental studies of ovulation rate in model species to all aspects of ovulation rates and genetic studies to identify quantitative trait loci or causative mutations affecting ovulation rates and more explicitly twinning rates in bovine species.
