This chapter discusses the uncertainty in estimating the impact of climate change on crop production using dynamic crop simulation models. We first briefly indicate the myriad ways in which climate change could impact agriculture in general but more particularly crop production. Then, we discuss the methods that are used to better understand and quantify the effect of climate and more specifically climate change on crop yield and quality. Since the most widely used and most flexible approach is crop modeling, the following sections, devoted to uncertainty, focus on this approach. Two different aspects of uncertainty are considered. We discuss the uncertainty in simulating the effect of climate change on crop production using models, measured as the variability between different simulation results for the same context. We also consider the uncertainty in the true value of production, given a simulated value, with uncertainty here as the discrepancy between true and simulated values. The final section contains suggestions for ways to better characterize and reduce uncertainty.