The chapter presents a systematic review of the basic foundations of biological evolution and research methodologies of evolutionary sciences. It describes major biological principles of evolution and specifically, how they work in the evolution of behavioral manifestations that resemble love and love-related behaviors. The author explains how evolutionary coexistence, competition, and cooperation of species and organisms produced the early forms of love in plants, birds, animals, and humans and how they navigate their relationships. Individuals in these species learned to love or hate others due to their evolutionary need to adapt their bodily functions and behaviors to their environment. Specific mechanisms of natural selection and sexual selection were evolutionarily valuable, supporting these evolutionary adaptations. The chapter introduces the three major types of love—sexual love, parent-infant love, and ingroup love between individuals—that the author comprehensively reviews in Chaps. 4 , 5 , and 6 . These types of love evolved as the adaptation mechanisms associated with major evolutionary functions: mating, caring for offspring, and interindividual relationships, such as kinship and ingroup cooperation.