This chapter reviews the behavioral ecology of aggression at the individual, population, and species levels, and identifies areas where lizards could be a suitable model system for addressing the influence of aggression on fitness more broadly. It describes aggression, patterns of aggressive behavior in lizards, and how studies of lizards have contributed to animal behavior theory more broadly. The chapter explains the mechanisms that underlie aggression and the emergence of aggression during development. It also reviews the evolutionary function of aggression in different contexts, including natural selection, sexual selection and social selection. The chapter discusses the potential implications of aggression for the ecological and evolutionary trajectory of populations. The influence of lateralization on aggression in lizards is, however, greatly understudied, and has the potential to contribute to researchers’ general understanding of lateralization more broadly. Research has demonstrated a positive association between elaborate aggressive displays and complex social structure.