DNA methylation is a type of chemical modification of DNA that can be inherited without changing the DNA sequence. As such, it is part of the epigenetic code and is the most characterised epigenetic mechanism. DNA methylation involves the addition of a methyl group to DNA -- for example, to the number 5 carbon of the cytosine pyrimidine ring. DNA methylation is probably universal in eukaryotes. In humans, approximately 1% of DNA bases undergo DNA methylation. In adult somatic tissues, DNA methylation typically occurs in a CpG dinucleotide context; non-CpG methylation is prevalent in embryonic stem cells.In plants, cytosines are methylated both symmetrically (CpG or CpNpG) and asymmetrically (CpNpNp), where N can be any nucleotide. The methylation status of specific cytosines can be determined using methods based on bisulfite sequencing. This book presents leading-edge research advances in the field.