Hydrangea is widely grown as an ornamental plant with a high commercial value owing to its appealing features. Although many cultivars exist as genetic resources, they cannot be effectively used for breeding due to insufficient information regarding their genetic relationships and breeding compatibility. Intraspecific and intergeneric hybridizations within the family, Hydrangeaceae present opportunities to create cultivars with desirable traits, however, breeding efforts in Hydrangea through interspecific hybridization have achieved limited success. The low viability of interspecific hybrids may be attributed to cytogenetic incompatibility between the species. Therefore, a deeper cytogenetic understanding of this genus could enhance future breeding programs. This review focuses on the importance of cytogenetic data in the breeding of hydrangeas and the different cytogenetic techniques utilized in this genus. Future directions for cytogenetic research in Hydrangea are also discussed.