Telomeres are in the spotlight of modern biology. Whether the subject at hand is cancer, gene regulation, organismal aging, or the cloning of mammals, much seems to depend on what happens at the ends of chromosomes. Because glamorous hypotheses often persist without experimental support, it is important to ask ourselves what we really know about telomeres. This was the topic of a recent meeting entitled ‘Telomeres and telomerase: cancer, aging, and genetic instability,’held at the Juan March Centre for International Meetings on Biology (Madrid). Leading scientists in the field of telomere biology presented their latest data in an intimate and friendly setting, as summarized below.