The purpose of this talk was to introduce some work done in collaboration  with Odile Pons (INRA, Jouy-en Josas) on the estimation of genetic differentiation in various contexts. Among the questions studied are the following: How to obtain confidence intervals for estimates of genetic differentiation (G_ST)? How to optimize sampling in order to obtain a better (more precise) estimate of differentiation? How to measure differentiation while taking into account information on the nature of the alleles (N_ST (sequence) or R_ST (microsatellite) data)? How to compare differentiation for ordered versus unordered alleles? What does this difference mean? How to get pairwise measures of differentiation? How to measure the differentiation of one population from the rest ? Is a given population important to conserve because it is variable or because it is distinct, or because of both? How to measure that? How to compare measures of allelic richness? What do statements such as: 10% of the diversity is distributed among populations' really mean?