Abstract: DNA methylation is one of the most important issues in epigenetics and plays a key role in plant research. This review introduces the progress of research into the mechanisms of DNA methylation, such as histone methylation, RNA-dependent DNA methylation, and demethylation, and discusses the functions of DNA methylation in plants, including defending the genome against transposons and regulating gene expression. The role of DNA methylation in transgene silencing and epigenetics is covered, as is approaches to study DNA methylation in plants, including high performance liquid chromatography, bisulphite sequencing, methylation- sensitive amplified polymorphisms, restriction enzyme digestion and Southern blot analysis.