My proposal seeks to explain the complex interplay between genetic and environmental causes of individual variation in substance use and the risk for abuse. Substance use is common. Substances like nicotine and cannabis have well-known negative health consequences, while alcohol and caffeine use may be both beneficial and detrimental, depending on quantity and frequency of use. Twin studies (including my own) demonstrated that both heritable and environmental factors play a role.My proposal on substance use (nicotine, alcohol, cannabis and caffeine) is organized around several key objectives: 1. To unravel the complex contribution of genetic and environmental factors to substance use by using extended twin family designs; 2. To identify and confirm genes and gene networks involved in substance use by using DNA-variant data; 3. To explore gene expression patterns with RNA data in substance users versus non-users; 4. To investigate biomarkers in substance users versus non-users using blood or urine; 5. To unravel relation between substance use and health by linking twin-family data to national medical databases.To realize these aims I will use the extensive resources of the Netherlands Twin Register (NTR); including both the longitudinal phenotype database and the biological samples. I have been involved in data collection, coordination of data collection and analyzing NTR data since 1999. With my comprehensive experience in data collection, data analyses and my knowledge in the field of behavior genetics and addiction research I will be able to successfully lead this cutting-edge project. Additional data crucial for the project will be collected by my team. Large samples will be available for this study and state-of-the art methods will be used to analyze the data. All together, my project will offer powerful approaches to unravel the complex interaction between genetic and environmental causes of individual differences in substance use and the risk for abuse.