Substance abuse has become a major challenge in secondary schools in Kenya. A study carried out in Kenya observed that 20% of adolescents aged between 12 and 22 years smoke cigarettes, 9% smoke bhang while 23% drink commercial beer and spirits. This is the age in which most youths are in schools and colleges. The aim of this study was to find out the incidence and extent of drug abuse among secondary school students in Nairobi Province, Kenya. This information is useful in developing specialized educational programs for drug abuse in Kenya. The objectives of this study were: to determine various substances commonly abused by students, assess the prevalence of substance abuse in secondary schools, determine the factors that contribute to substance abuse, students'' behaviour related to substance abuse and identify the factors that influence students to abuse drugs. An expost-facto research design was employed. The study population comprised of all public secondary schools in Nairobi Province. The sampling was done using stratified sampling to obtain different categories of schools, and the status of each school.