Stress testing, at its most general level, is an investigation of the performance of an entity under abnormal operating conditions. The authors focus on one set of entities—the Canadian banking sector—and investigate losses in the loans portfolio of this sector as a function of changing circumstances in the different industries in which these loans reside. These circumstances are characterized by means of one summary measure—sectoral probabilities of default—and this measure is modelled as a function of macroeconomic variables. Using this model, the authors assess the interrelationship between the macroeconomic environment and sectoral defaults, and perform a series of stress tests under different scenarios that are thought to be most pertinent to Canada. The tools underlying the authors' analysis are general and can be applied to other countries, as well as to other macroeconomic scenarios.