The sequence organisation of the telomeric regions is extremely similar for all eukaryotes examined to date. Subtelomeric areas may contain large sequence arrays of middle repetitive, complex elements that sometimes have similarities to retrotransposons. In between and within these complex sequences are short, satellite-like repeats. These areas contain very few genes and are thought to be organised into a heterochromatin-like domain. The terminal regions almost invariably consist of short, direct repeats. These repeats usually contain clusters of 2-4 G residues and the strand that contains these clusters (the G strand) always forms the extreme 3 -end of the chromosome. Thus, most telomeric repeats are clearly related to each other which in turn suggests a common evolutionary origin. A number of different structures can be formed by single-stranded telomeric G strand repeats and, as has been suggested recently, by the C strand. Since the main mechanism for the maintenance of telomeric repeats predicts the occurrence of single-stranded extensions of the G strand, the propensity of G-rich DNA to fold into alternative DNA structures may have implications for telomere biology,