In this chapter, we discuss the evolution of complex traits influenced by multiple genes and environmental factors. Using human height as an example, we demonstrate the relationship between the number of genes affecting a trait and the continuous variation in trait values observed across individuals in a population. Then, we explore how the relative contributions of genetic and environmental factors to trait variation can be disentangled, leading to the concept of trait heritability. We also describe how complex traits respond to selection and how the short-term response can be predicted from the breeder’s equation. Finally, we discuss complex diseases such as type II diabetes and the potential of genomics, particularly large-scale genome-wide association studies, for identifying the genes underlying variation in disease risk between individuals.