Telomeres are specialized DNA-protein structures present at the ends of all linear chromosomes and are characterized by (TTAGGG)(n) hexanucleotide repeats and associated proteins. Telomere length has been implicated in cell survival and replicative capacity of dividing somatic cells. In the absence of an active compensatory mechanism, telomere lengths shorten as a consequence of proliferation, both in vitro and in vivo. However, this loss of telomeric repeats can be compensated and telomere length maintained by the enzyme telomerase, which is capable of adding (TTAGGG) repeat sequences to the ends of telomeres. This unit describes methods that are used for the measurement of telomere length and telomerase activity in human and murine cells.