Abstract Tuberculosis (TB) is a leading causes of global mortality. Nearly one-third of the world's population is infected with M. tuberculosis, an estimated 10.4 million new cases of TB develop annually, and 1.4 million persons die from the disease. The HIV pandemic and TB/HIV co-infection have fueled escalating TB incidence, and complicated TB management and control. The emergence of multi-drug resistant (MDR) and extensively drug resistant (XDR) TB has exacerbated the threat to public health and created a renewed sense of urgency to control the disease. Treatment of MDR-TB leads to emergence of additional drug resistance in 6-20% of patients on treatment. We propose to investigate the causes of emergence of mycobacterial drug resistance in an observational cohort study, the Predictors of Resistance Emergence Evaluation in MDR-TB Patients on Treatment (PREEMPT) Study. This proposal takes advantage of two existing TB cohort studies in India and Brazil (Regional Prospective Observational Research for Tuberculosis: RePORT India and RePORT Brazil), co-funded by NIAID and the Indian and Brazilian governments, respectively. In the PREEMPT study, we will enroll 400 patients with MDR-TB in India and Brazil over a 24-month period and follow them prospectively for 3 years. The proposal has the following Specific Aims: Aim 1: To determine whether low serum antimycobacterial drug concentrations contribute to the emergence of drug resistance in MDR TB patients, and whether HIV seropositivity is a risk factor for low serum drug concentrations and/or the emergence of resistance. Aim 2: To determine the contribution of increased DNA mutation (caused either by an intrinsic property of the M. tuberculosis strain or by FQ treatment) to clinical emergence of drug resistance in patient isolates. Aim 3: To determine whether mutations responsible for drug resistance can be detected early during treatment, when an intervention to prevent XDR-TB might be possible. This proposal builds on an existing TB research framework to study mechanisms of emergence of TB drug resistance during MDR-TB treatment. It is very likely that similar mechanisms will explain emergence of TB drug resistance during treatment for drug-susceptible TB. Each of the proposed mechanisms of emergence of resistance that we will investigate has direct implications for new regimen designs, new drug administration algorithms and new drug susceptibility monitoring strategies that could prevent emergence of resistance in patients with TB with and without HIV.