Telomeres are the components of chromosome ends that provide stability and allow the complete replication of the ends. Telomere length is maintained by a balance between processes that lengthen and those that shorten telomeres. Telomerase is a ribonucleoprotein polymerase that specifically elongates telomeres. In human cells telomere length is not maintained and telomerase is not active in some tissues. In tumors, however, telomerase is active and may be required for the growth of cancer cells. Thus understanding telomerase and telomere length regulation may help us understand tumor progression. Evidence from various organisms suggests that several factors influence telomere length regulation, such as telomere binding proteins, telomere capping proteins, telomerase, and DNA replication enzymes. Understanding how these factors interact to coordinate the regulation of telomere length will allow a more complete understanding of telomere function in the cell.