I investigated stem cell dynamics in normal human colon by detecting somatic variants affecting X-linked and autosomal genes using immunohistochemistry. Applying neutral clonal marks to a large cohort of patients to interpret age-related trends in clone frequencies has established the baseline stem cell dynamics of the tissue. Analysis of a number of new clonal marks showing biased behaviours suggests that different gene-specific mutations can subvert constraints resulting from the tissue architecture in different ways. In respect of intra-gland competition of stem cell derived clones, a disadvantage is observed for the histone modifier HDAC6, while at the other end of the spectrum, loss of the cohesin member STAG2 strongly advantages affected stem cells. Subsequent clonal expansion beyond the boundary of a single crypt is recognised by clones occupying multi-crypt patches. Quantification of such events allows the rate of lateral expansion for different mutations to be measured. Moderate effects were found for PTEN, p53 and STAG2, while mutations in the histone demethylase KDM6A generate very large areas of mutant epithelium in aged humans. Further, targeted sequencing revealed dramatic expansion of KRAS-mutant clones in histologically normal colon. Patches may arise from crypt fission and fusion events. Using a clonal mark based on mild periodic acid-Schiff staining, the neutral crypt fission and fusion rates were quantified. Against this baseline, it was found that KDM6A-mutant clones expand mainly by fission, while fusion remains at homeostatic levels. The emerging picture is that of the aged human colon consisting of a mosaic of different mutations. The work presented in this thesis offers detailed insights into the rates at which different gene-specific mutations arising in colonic stem cells can become fixed within individual crypts and undergo subsequent lateral expansion through crypt fission and fusion events. This defines the timeframe taken for cancer drivers to achieve high mutant allele burden within the tissue, which can serve as a basis for cancer prevention strategies.