This chapter describes methods for inferring the time to the most recent common ancestor of a population, given DNA sequence data from a sample of individuals from the population. The coalescent is used to model the ancestral relationships between the individuals in the population, allowing for deterministic fluctuations in population sizes through time. The data are summarized by using the number of segregating sites, or variable bases, in the sample sequences. The conditional distribution of the time to this most recent common ancestor, given the number of segregating sites, is found by a Monte Carlo method.