It is widely documented that knowledge of sexually transmitted infections (STIs) and modern contraceptives should translate into a sexual behaviour that prevents STIs and unwanted pregnancy. This study was carried out to explore senior high school (SHS) students (girls) sexual behavior in relation to their knowledge about STIs and modern contraceptives. The study is based on a sample of 180 girls from three SHS in the Akuapem North Municipality, Ghana. Chi square test was performed to find out the relationship between knowledge about modern contraceptives, STIs and sexual behaviour. Results indicate that both knowledge of modern contraceptive and STIs are high and that knowledge of contraceptives and STIs influence utilization. However, adolescent girls utilize emergency contraceptives compared to condom. Contraceptive method preferred is different from methods used; and method used is different from intended reasons for contraception.