Multidrug-resistant bacteria are a global threat to public health and the economy. Studies in model organisms suggest compensatory evolution and epistatic interactions between drug resistance-conferring mutations are important drivers of drug resistance. However, the relevance of these factors for the emergence and transmission of human pathogenic bacteria has not been established. To bridge the gap between laboratory experimentation and epidemiology, we are using a multidisciplinary approach focusing on Mycobacterium tuberculosis, the etiologic agent of human tuberculosis (TB). We are combining experimental evolution and fitness assays with comparative and functional genomics, and population-based molecular epidemiology to i) identify and characterize compensatory mutations in M. tuberculosis resistant to different anti-TB drugs, ii) detect epistasis between drug resistance-conferring mutations in different strain genetic backgrounds, and iii) investigate the effect of drug resistance-conferring mutations, compensatory mutations, and their epistatic interactions on the M. tuberculosis phenotype. Through its multidisciplinary nature, this project will simultaneously test predictions from ecological theory and experimental models, and generate new insights into the biology and epidemiology of multidrug-resistant TB.