The telomerase reverse transcriptase has an essential role in telomere maintenance and in cancer biology. Progress during the last year has revealed the three-dimensional architecture of both human and ciliate telomerase at about 25Å resolution, obtained using single particle electron microscopy (EM). The structural analysis of the two holoenzyme complexes isolated from cells shows that whilst the ciliate telomerase is monomeric, the human telomerase is dimeric and only functional as a dimer. We critically discuss the approaches taken to assign the location of protein and RNA subunits, as well as fitting the crystal structure of the catalytic protein subunit in the medium resolution EM density maps. Comparison of the two structural interpretations reveals not only a common RNA/reverse transcriptase core, but also significant differences due to different RNA subunit size and protein composition. These differences suggest that the oligomeric state and subunit composition of telomerase in evolutionary distant organism have evolved.