An understanding of the inheritance of quantitative traits, those with a continuous phenotype, was first established in the early 1900s. This was instrumental for breeding because quantitative genetic theory provides the basis for the development of methods which can be used to increase the rate of genetic improvement, referred to as “genetic gain,” within a breeding population over time. Today, the concept of genetic gain and its basis in quantitative genetics is often not well understood among crop breeders and scientists, often resulting in inefficient or ineffective crop improvement efforts. This chapter aims to provide clarity on genetic gain to help those engaged in crop improvement to take actions that will enable them to be more successful. To do so, a thorough introduction to genetic gain and the population improvement cycle is provided along with a review of selection techniques essential for breeding. Next, I demonstrate why genetic improvement on a population basis is needed facilitate variety development. In order to show that the genetic gain is tractable, the theory behind genetic gain and its prediction is explained, followed by a discussion on realized genetic gain including a review of methods that can be used for its estimation. Lastly, guidance is given on how to improve rates of genetic gain in applied breeding programs.