This chapter reviews the definitions of relational aggression and other subtypes of aggression that are often studied in the developmental sciences. Specifically, definitions of relational, physical, indirect, social, verbal, nonverbal, proactive, and reactive aggression are provided. The modes, forms, functions, and contexts of aggression are reviewed, with a focus on relational aggression. Attention is given to other related constructs such as assertion, rough and tumble play, and social dominance, which should be considered and ruled out when studying subtypes of aggression. A definition and important considerations for the study of relational bullying are provided, and a brief discussion of the contexts of online or electronic aggression is given. Future directions and unanswered questions are raised.