Microsatellites are mutable repeated DNA sequences, e.g. ATATATATATAT = (AT)6, that are common in the genomes of higher organisms. Here I discuss their evolutionary dynamics. 1)When is a DNA repeat sequence a microsatellite? (AT)6 is generally presumed to be a microsatellite, but what about (AT)4 or (AT)2? Genomic sequence data from yeast, nematode and humans provides an insightful answer. 2) Why are orthologous microsatellites from distantly related sequences similar in length? The mutability of microsatellites has been shown to be dependent on the length of the repeat sequence. Theoretical investigation suggests that this length dependence provides a general (and entirely neglected) explanation for the narrow range of the lengths of microsatellites.