Histone methylation has important roles in regulating transcription, genome integrity and epigenetic inheritance. Historically, methylated histone arginine and lysine residues have been considered static modifications because of the low levels of methyl-group turnover in chromatin. The recent identification of enzymes that antagonize or remove histone methylation has changed this view and now the dynamic nature of these modifications is being appreciated. Here, we examine the enzymatic and structural basis for the mechanisms that these enzymes use to counteract histone methylation and provide insights into their substrate specificity and biological function.