DNA methylation is an essential chemical modification of the DNA involved in gene regulation, cellular differentiation and chromatin biology. The process has been described by a classical model of maintenance of DNA methylation at CpG sites 40 years ago. However, substantial experimental evidence from the past decade indicates that this simple model needs to be revised. We argue in a new review paper that DNA methylation can be described by a dynamic stochastic model, in which the DNA methylation at each site is determined by the local activity of DNA methyltransferases, DNA demethylases and the DNA replication rate. DNA methylation is controlled by the network of chromatin marks which influecne the targeting and regulation of these enzymes. This novel model explains the roles and biological effects of DNA methylation in health and disease.