The chapter introduces the concept of optimization in bird migration, emphasizing that it involves minimizing costs or maximizing benefits. The use of optimization methods in bird migration research is not new, with early theories by Pennyquick and Tucker providing a foundation for interpreting flight adaptations. The chapter discusses three critical costs and benefits for migratory birds: speed, energy economy, and predation risk. It argues that different optimization criteria lead to distinct migratory adaptations, such as fat deposition strategies, flight behavior, and habitat selection. The chapter also presents a mathematical model to describe the relationship between fat load and flight distance, showing that the potential flight distance decreases with increasing fat load due to higher flight costs. This relationship is illustrated in a figure, highlighting how the utility of additional fat decreases as fat reserves increase.The chapter introduces the concept of optimization in bird migration, emphasizing that it involves minimizing costs or maximizing benefits. The use of optimization methods in bird migration research is not new, with early theories by Pennyquick and Tucker providing a foundation for interpreting flight adaptations. The chapter discusses three critical costs and benefits for migratory birds: speed, energy economy, and predation risk. It argues that different optimization criteria lead to distinct migratory adaptations, such as fat deposition strategies, flight behavior, and habitat selection. The chapter also presents a mathematical model to describe the relationship between fat load and flight distance, showing that the potential flight distance decreases with increasing fat load due to higher flight costs. This relationship is illustrated in a figure, highlighting how the utility of additional fat decreases as fat reserves increase.