Background: Many of the common causes of morbidity and mortality for today and tomorrow can be prevented by reducing certain categories of behavior among youth with health promotion against tobacco use, alcohol and substance use being one of them. Objective: The objective of the study was to study the magnitude, patterns and various determinants of substance abuse disorders prevalent in the youth population in a slum of Mumbai Materials and Methods: This was a cross-sectional study conducted in Malvani slum area of Mumbai among 540 youth between 15 to 24 years of age. Two stage systematic random sampling method was used with individual household being the sampling unit. Results: 43.5% of the study participants had habit of abusing one or more than one substance. 22.6% of female participantsalso indulged in substance abuse. 28.9% of all substance abusers were consumers of alcohol. The habit of substance abuse began early, with 14% beginning before 13 years of age. Majority (78.3%) had taken to substance abuse in adolescence. Around 1/5th of the monthly wages in 40% of substance abusers was spent on substance abuse. Conclusion: The gap between knowledge and practice was marked in case of substance abuse. Alternative approaches will have to be tapped to find out the effective strategies to prevent the youth from indulging in substance.