This article deals with methods for identifying as well as stressing risk concentrations in credit portfolios, in particular concentrations caused by large exposures to a single sector or to several highly correlated sectors. We present a general and yet computationally efficient framework for implementing stress scenarios in a multi-factor credit portfolio model and illustrate the proposed methodology by stressing a large investment banking portfolio. Although the methodology is developed in a particular factor model, the main concept-stressing sector concentration through a truncation of the distribution of the risk factors-is independent of the model specification. We introduce the concept of Factor Concentration that formalizes the proposed approach and analyze its mathematical properties.