Somatic mutations acquired in healthy haematopoietic stem cells (HSCs) are an inevitable consequence of ageing. Some of these mutations disrupt the balance of stem-cell self-renewal and lead to detectable clonal expansions termed clonal haematopoiesis (CH). These detectable mutations provide rich information for understanding stem cell population sizes and dynamics. In this talk I will briefly describe how quantitative analyses of clone-size distributions from large-scale sequencing data obtained from hundreds of thousands of individuals can be used to provide estimates of stem cell population sizes and division rates as well as the biases in self-renewal caused by specific genetic alterations in blood. Our analyses reveal that most events driving clonal expansions in blood occur outside of canonical cancer-associated genes suggesting many “missing” driver mutations yet to be discovered. I will show how these missing drivers are able to resolve a deeply puzzling observation about the age-dependence of CH and outline ongoing work to identify possible epigenetic drivers of these clonal expansions.