Milk fat contains many nutrients necessary for humans, including fat-soluble vitamins, energy, and bioactive lipids. It is important to understand the genetic basis for milk-fat composition in cows’ milk. Knowledge of these genetic parameters can be used to predict how different traits will respond to genetic selection. The predicted response in traits, including trade-offs between different traits, plays an important role in designing breeding schemes in dairy cattle. This chapter reviews the genetic variation in milk-fat composition between cows and looks into the genetic correlations with other traits.