Various breeding strategies to improve dairy cattle production, conformation, health and, more recently, efficiency, have been developed. However, as selection intensity within these finite populations has increased, so has the relatedness within individual populations. This chapter looks at inbreeding and genetic diversity in the Holstein breed. It discusses the forces affecting genetic diversity within the breed, including drift, selection, migration, and mutation. Assessing measures of relatedness using different sources of information available for Holsteins (additive genetic relationships, genomic relationships, and other measures of inbreeding) are described. Inbreeding and its effects on phenotypes, including positive effects of inbreeding, inherited defects, and inbreeding depression for quantitative traits, are then explored. Exploration of population measures of inbreeding, such as the effective population size and the rate of inbreeding leads into a discussion on how to manage genetic diversity in Holsteins. The chapter concludes with a case study on Holstein cattle in Canada.