We provide a conceptual primer for sexual selection and conflict, mating systems, and socio-sexual behaviors and patterns among animals, largely with mammalian and cetacean examples. The important roles of mate choice are discussed (including female choice) and the occasional fluidity of sexual roles. An overview of topics pertinent to sex and behavior is described, including evolutionary drivers (the concept of “why sex after all?”) followed by general mammalian and cetacean mating strategies and tactics. We describe mating systems (monogamy, polygyny, polyandry, polygynandry) with the present understanding that most cetaceans do not have monogamous or polyandrous mating systems. The primer includes brief introductions to historical knowledge and highlights emerging areas of research within the field of sex in cetaceans, with context for other chapters of this book. As part of overall sexual behavior, alloparental care, female reproductive senescence, and non-procreative behaviors including homosexual and necrocoital mating are also introduced.