This study, using the Buss-Perry Aggression Questionnaire (AQ) (Buss and Perry, 1992), the Rosenburg Self-Esteem Scale (RSE) (Rosenburg, 1965), the Eysenck Personality Revised Questionnaire (JEPQ-S) (Eysenck, 1985) and a questionnaire relating to sports participation, along with a few other demographic questions relating to gender and age, aims to investigate the relationship between aggression and gender, personality, participation in sports and self-esteem. The first hypoFinal Year Project states that there will be a significant difference in levels of aggression between males and females. The second hypoFinal Year Project states that there will be a significant relationship between personality and levels of aggression. HypoFinal Year Project three examines levels of aggression in relation to sports participation and the fourth hypoFinal Year Project looks at the relationship between self-esteem and aggression. 183 adolescents, 78 female and 105 male, from 2 post primary mixed sex schools in Co. Monaghan participated in this study. Participants were between 12 and 19 years old with a mean age of 14.19. Results show that personality has a strong significant main effect on aggression. Gender shows a weak significant relationship to physical aggression and self-esteem shows a strong significant effect on hostility.