Exposure of populations to antibiotics is a condition for the emergence and the diffusion of bacteria resistant to antibiotics. Analysis of epidemiological causality between antibiotic exposure and expansion of antibiotic resistance in human pathogens requires the study of the basis for the emergence and the subsequent epidemic spread of the strains. Emergence of antibiotic resistance under the selective pressure of antibiotics is unavoidable. However, the rapidity of the emergence depends on several parameters including the biochemical mechanism and the genetic support of the resistance and on the couple bacteria/antibiotic considered. Several of these parameters may be studied and measured in vitro. In vivo, relationship between antibiotic exposure and risk for infection or colonisation by pneumococci resistant to beta-lactams or macrolides and methicillin-resistant staphylococci are among the best studied models. In particular for pneumococci, quantitative relationship between antibiotic consumption and resistance, including geographical correlations have been shown but do not allow to establish any causality relationship. Overall, there is no doubt on the question of the impact of antibiotic exposure and the risk of antibiotic resistance. However, the quantification of the risks remains to be accurately studied.