<p>Understanding the early stages of carcinogenesis requires detailed insight into the abnormalities present in normal cells before cancer onset. In the past, it was difficult to analyze genomic abnormalities in small clones in normal tissues. However, recent technological advances in genomic analysis have shed light on the process of accumulation of somatic mutations in normal cells, which is driven by factors such as aging and environmental influences. Even in normal tissues, clones that have acquired driver mutations—either directly contributing to carcinogenesis or adapting to specific pathological or genetic backgrounds—are frequently selected, leading to clonal expansion. Normal cells undergo clonal evolution into cancer cells over several decades, with the initial acquisition of a driver mutation occurring in early life. Here this review presents recent findings concerning the accumulation of somatic mutations in normal cells, acquisition of driver mutations and clonal evolution toward cancer.</p>