Telomerase, a eukaryotic ribonucleoprotein, can help to maintain the length of telomeres in human cells by adding multiple TTAGGG repeats by its integral RNA. The activity of telomerase has been found to be overexpressed in most human tumour cells, hence the activity of telomerase has been considered as an important tumour marker, which makes the detection of telomerase extremely important. The early developed methods for telomerase activity measurements were performed in vivo, using cell extracts from thousands of cells. However, these methods could not provide information on telomerase activity in single cells. In this case, in situ analysis of intracellular telomerase is of great significance to the study of its heterogeneity and distribution at the single-cell level. The in situ study of telomerase belongs to a cross-disciplinary research field including chemistry, materials science, biology and clinical medicine. With the rapid development of high-end technical measures, the in situ detection of intracellular telomerase has been greatly promoted. This chapter summarizes some of the recent developments in in situ analytical methods for intracellular telomerase activity. These methods have achieved the in situ tracking of intracellular telomerase activity with good performance and stability and can also be used for distinguishing tumour cells from normal cells. It is believed that these methods will provide powerful and convenient tools for cancer diagnosis and contribute to the screening of telomerase-targeting drugs. More exciting developments are still to come in the near future, for example, defeating cancer and life extension.