Dissertation (MSc Public Health)Background: The substance abuse among adolescentsespecially secondary school students remains a significant public health issue. The substance abuse has strong correlation with delinquency behaviour. This study intended to determine psychosocial determinants of delinquency behaviour and substance abuse among adolescents in secondary schools at Kongwa District and Dodoma City. Methods and Materials Analytical, cross-Sectional study design with quantitative approach self-administered structured questionnaires which was adopted and used to collect data.The study populations‘ for this study was secondary school adolescents. The sample of 422 of secondary school adolescents was involved in the current study. Multistage sampling technique was used to select secondary schools. Statistical Package for Social Sciences (SPSS) software version 20 was used to analyze the data. Results: The Majority (48.6%) of respondents were aged between13 and 15 years. Themean age and SD were 16 and 9.911) respectectively.Malerespondents comprised, (51.2%) whereas female respondents comprises (48.8%.Prevalence of substance abuse were (26.3%).The prevalence of delinquency behaviour was (70.1%), Mean of delinquency was 1.22.Peak age was13-15years (33.8%), boys were (36.96%) girls were (33.2).The correlation between delinquency behaviour and substance abuse was positive(r=0.202, p=0.000). Conclusion: Substance abuse and delinquency behaviour are serious problem among secondary school adolescents in Tanzania. These problems seem to increase every day.Key word: Substance abuse, Delinquency, adolescents, Secondary school.