Genetic diversity in dairy cattle underpins long-term productivity, resilience, and adaptability. Although structured breeding and genomic selection have substantially accelerated genetic gain in traits such as milk yield and feed efficiency, these advances have simultaneously reduced effective population size, increased inbreeding, and eroded genetic variability. This chapter synthesizes pedigree- and genomic-based approaches for quantifying and managing genetic diversity, including effective population size, genomic relationships, and runs of homozygosity. The consequences of diversity loss are examined, with emphasis on inbreeding depression affecting fitness-related traits in both females and males, and on emerging evidence identifying male fertility as a diversity-sensitive bottleneck. The role of small, local, and indigenous breeds as reservoirs of adaptive genetic variation is highlighted. Sustainable breeding strategies, which include optimal contribution selection, region-aware mate allocation, genomics-informed mating constraints, germplasm exchange, and conservation programs are discussed as practical tools for balancing genetic gain and diversity.