This chapter provides a comprehensive introduction to the field of genetics applied to sports, exploring the fundamental concepts and current applications. It begins by explaining the basic structure and function of DNA, genes, alleles, and the process of gene expression, highlighting how genetic variations, particularly single nucleotide polymorphisms (SNPs), contribute to individual differences. The text delves into the complex interplay between genetic inheritance and environmental factors in shaping athletic traits such as strength, speed, endurance, and susceptibility to injury, emphasizing the polygenic nature of sports performance. Key genes like ACTN3, ACE, PPARGC1A, COL5A1, and others associated with muscle function, cardiovascular efficiency, energy metabolism, tissue integrity, and response to training are discussed with specific examples of relevant SNPs (e.g., rs1815739, ACE insertion/deletion (I/D), rs8192678, rs12722). The chapter also covers the evolution and current state of genetic testing in sports, including methodologies (SNP arrays, NGS, qPCR), interpretation challenges, and the crucial role of qualified professionals. Finally, it addresses the significant ethical considerations, international guidelines, and concerns surrounding the use of genetic information in sports, advocating for a responsible, evidence-based, and athlete-centered approach that uses genetics as a tool for personalization and optimization, rather than deterministic prediction or exclusion.