Telomerase is the enzyme responsible for the maintenance of telomere length by adding guanine-rich repetitive sequences. Its activity can be seen in gametes, stem cells and tumor cells. In human somatic cells the proliferative potential is limited, reaching senescence after 50-70 cell divisions, because the DNA polymerase is not able to copy the DNA at the ends of chromosomes. By contrast, in most tumor cells the replicative potential is unlimited due to the maintenance of the telomeric length given by telomerase. Telomeres have additional proteins that regulate the binding of telomerase, likewise telomerase associates, with a protein complex that regulates its activity. This work focuses on the structure and function of the telomere/telomerase complex and how changes in its behavior lead to the development of different diseases, mainly cancer. Development of inhibitors of the telomere/telomerase complex could be a target with promising possibilities.