The rapid progress in deciphering the processes of covalent histone modifications has broadened our understanding of transcriptional regulation. Histone lysine methylation, along with DNA methylation, establishes the framework for long-term epigenetic maintenance. Recent studies of the mechanisms of specific histone lysine methylation have revealed a complex process that controls aspects of short- and long-term transcriptional regulation, in addition to the propagation of bulk chromosome structure and stability. In this article, we review the functional properties of histone lysine methylation and the enzymes that catalyze this covalent modification.