Traditionally, human aggression was thought to be almost entirely the preserve of males. This is because males usually displayed overt physical and verbal behaviour that is noticed. Over the last two decades, researchers have broadened their conceptualisations of aggression to include more indirect forms. This research revealed that girls, especially in the teenage years, exhibit more indirect aggression than boys and that this form of aggression is very hurtful to girls. Interventions are therefore important but because of the covert nature of indirect aggression, they are proving elusive. This chapter summarises the research on indirect aggression, including work of the author, and the recent efforts to find effective interventions. Interventions will need to take account of the nature of teenage girls' friendships, the functions of indirect aggression, and girls' own relational strengths and language and social skills.