Introduction 3. What are telomeres? 4. Telomere shortening occurs naturally in human somatic cells 5. Telomerase adds telomeres to the end of linear chromosomes 6. Telomere dysfunction: causes and consequences 6.1. Short telomeres trigger DNA damage signals that lead to cell cycle arrest in normal human cells 6.2. Telomere dysfunction triggers genomic instability in checkpoint defective cells 7. Role of telomere-mediated genomic instability in carcinogenesis 8. Heterogeneity in telomere lengths on individual human chromosomes and its impact on cancer development 9. Telomerase activation alleviates chromosome instability during immortalization and tumorigenesis 10. Conclusions and future perspective