Genetic and epigenetic intra-tumoral heterogeneity cooperate to shape the evolutionary course of cancer. Chronic lymphocytic leukaemia (CLL) is a highly informative model for cancer evolution as it undergoes substantial genetic diversification and evolution after therapy. The CLL epigenome is also an important disease-defining feature, and growing populations of cells in CLL diversify by stochastic changes in DNA methylation known as epimutations.