Security dialogs warn users about security threats on their computers; however, people often ignore these important communications. This paper explores the links between warning dialog design and user understanding of, motivation to respond to, and actual response to computer security warnings. We measured these variables through a 733-participant online study that tested a set of four existing computer security warnings and two redesigned versions of each across low- and high-risk conditions. In some cases our redesigned warnings significantly increased participants’ understanding and motivation to take the safest action; however, we were not able to show that participants’ responses were differentiated between low and high risk conditions. We also observed that motivation seemed to be a more important predictor of taking the safest action than understanding. However, other factors that may contribute to this behavior warrant further investigation.