Telomeres are the terminal portions of chromosomes and consist of the repeated nucleotide sequence TTAGGG. Chromosomes lose a small amount of telomeric deoxyribonucleic acid (DNA) after each cell replication. A hypothetical function of telomeric DNA is to allow for a finite number of cell divisions without loss of functional genes. A second proposed function of telomeric DNA is to prevent undesirable interactions between chromosomal ends and cellular repair enzymes. In cells that maintain a proliferative capacity, such as stem cells and cancer cells, telomere length is maintained by the reverse transcriptase, telomerase. Virtually every major human malignancy has been evaluated for telomerase activity and approximately 80% to 90% have demonstrated the presence of telomerase. In this article we review the current assays available for telomerase detection and discuss their relative strengths and limitations.