Age-related effects in the stem cell population. Stem cell populations, and their progeny, are subjected to age-effects that are related to the decline in tissue regeneration and homeostasis and consecutive onset of chronic human diseases, such as neurodegenerative disorders, heart failure, and diabetes. Deregulation of stem cell function or decrease of their population, due to cell senescence and apoptotic death, are the main factors for these age-related observations. Furthermore, malignant transformation of stem/progenitor cells may occur, leading to tumour formation and carcinogenesis. Indeed, the aging process is associated with the accumulation of genetic/epigenetic alterations within the stem/progenitor cell populations, together with telomeric DNA reduction, DNA repair deficiency, and chromosome rearrangements that may induce this process. During time, endogenous (e.g. ROS, RNS, spontaneous hydrolysis, alkylation, replication errors, telomere shortening, etc.) and exogenous (e.g. UV light, chemical, chemotherapeutic and radioactive compounds, X-rays, ionizing radiation, etc.) damage may occur, thus affecting the stem cell population.