Understanding the genetic underpinnings of substance use disorders and other addictive disorders promises to develop more effective and targeted prevention strategies, design treatments according to individual genetic profiles, and explore the biological and environmental interactions that contribute to these complex conditions. Genome-wide association study (GWAS) leverages large-scale genotype data and identifies genetic variants that increase susceptibility to substance use disorders. In this chapter, we will highlight a number of key genes that have been frequently replicated in different types of substance use disorders. We also discuss the shared genetic architecture that underlies multiple forms of addiction, illustrating how genetic predispositions can overlap across different substances. Further, the clinical application and consideration of polygenic score, genetic testing, and drug repurposing analysis is discussed. While these methods provide promising opportunities, they are not without limitations. Environmental factors, polygenic effects, and ancestry-specific effects should also be taken into consideration. This chapter aims to provide a comprehensive overview of the genetic studies of substance use disorders and behavioral addictions, highlighting their potential, limitations, and the future directions necessary for improving addiction treatment and prevention strategies.