This chapter discusses the concepts of orthologs and paralogs in evolutionary genomics, emphasizing their definitions, subtypes, and evolutionary implications. Orthologs are genes derived from a single ancestral gene in the last common ancestor of the compared species, while paralogs are genes related via duplication. The chapter explores the historical development of these concepts, their importance in understanding gene evolution, and the challenges in identifying them accurately. It highlights the impact of lineage-specific gene loss, horizontal gene transfer (HGT), and gene fusion on orthology and paralogy relationships. The chapter also reviews computational methods for identifying orthologs and paralogs, including phylogenetic analysis and sequence similarity-based approaches. It discusses the prevalence of one-to-one orthologous relationships and the role of orthologous clusters in evolutionary and functional genomics. Additionally, it examines the molecular clock hypothesis and the relationship between gene duplication and evolutionary rates, as well as the implications of xenologous and pseudoorthologous genes. Overall, the chapter underscores the critical role of orthologs and paralogs in understanding the evolutionary history and functional diversity of genes across different species.This chapter discusses the concepts of orthologs and paralogs in evolutionary genomics, emphasizing their definitions, subtypes, and evolutionary implications. Orthologs are genes derived from a single ancestral gene in the last common ancestor of the compared species, while paralogs are genes related via duplication. The chapter explores the historical development of these concepts, their importance in understanding gene evolution, and the challenges in identifying them accurately. It highlights the impact of lineage-specific gene loss, horizontal gene transfer (HGT), and gene fusion on orthology and paralogy relationships. The chapter also reviews computational methods for identifying orthologs and paralogs, including phylogenetic analysis and sequence similarity-based approaches. It discusses the prevalence of one-to-one orthologous relationships and the role of orthologous clusters in evolutionary and functional genomics. Additionally, it examines the molecular clock hypothesis and the relationship between gene duplication and evolutionary rates, as well as the implications of xenologous and pseudoorthologous genes. Overall, the chapter underscores the critical role of orthologs and paralogs in understanding the evolutionary history and functional diversity of genes across different species.