Math-education reformers encourage the incorporation of mathematical modeling activities into K-12 curricula. Many of the purported educational benefits derive from the authenticity of the activities-how well they reflect the everyday and occupational mathematical practices of adults. But a paucity in the literature of observational descriptions of adult modeling behavior has made it difficult to judge the authenticity of classroom activities and their potential to prepare students for out-of-school problem solving. The ethnographic study reported here investigated the everyday problem-solving activity of structural engineers in practice. Modeling was found to be central to and ubiquitous in the engineers' work, giving rise to some of their greatest intellectual challenges. These engineers use, adapt, and create models of various representation forms and degrees of abstraction. Two major challenges of modeling emerged: understanding inaccessible phenomena and keeping track of models. These challenges, and the nature of engineering models themselves, are not well reflected in the modeling tasks typically prescribed for the K-12 classroom, which will likely limit their educational benefits.