The availability of nucleotide and amino acid sequences sampled at different points in time has fostered the development of new statistical methods that exploit this temporal dimension. Such sequences enable us to observe evolution in action and to estimate the rate and magnitude of evolutionary processes through time. Populations for which such studies are possible measurably evolving populations (MEPs) - are characterized by sufficiently long or numerous sampled sequences and a fast mutation rate relative to the available range of sequence sampling times. The impact of sequences sampled through time has been most apparent in the disciplines of RNA viral evolution and ancient DNA, where they enable us to estimate divergence times without paleontological calibrations, and to analyze temporal changes in population size, population structure and substitution rates. Thus, MEPs could increase our understanding of evolutionary processes in diverse organisms, from viruses to vertebrates.