The chapter describes the diversity of multifaceted, fussy, and elusive meanings of love that are dispersed in scholarly literature. The author examines what scientists think and do when they research love and love-related constructs, presenting the basic concepts of love in a well-organized system. The sections of this chapter highlight the importance of defining love conceptually and operationally. The chapter briefly characterizes the traditional methods to study love and the challenges these methods encounter when they study the biological evolution of love and its origins in animals and early humans. Evolutionary studies of love bring both challenges and opportunities for our understanding of love.