Recent years have seen an upsurge in terrorist activity. Many studies have shown that the United States' transportation infrastructure is vulnerable to attack. This is especially true for bridges. While the probability of an attack's occurrence is very low, the structural response and consequences could be substantial. Bridge owners need a way to assess the vulnerability of their facilities, in order to make well-informed decisions regarding risk management and mitigation. This thesis proposes a method to evaluate the effects of a man-made attack using vehicle-bound conventional explosives on a highway bridge. The body of knowledge in performance-based seismic design is extended to blasts, using performance criteria to characterize structural response. The expected value of three major consequences is estimated: structural damage, casualties, and downtime. Probability distributions are used throughout to account for uncertainty. Three case studies apply the proposed method to different functional bridge types of an existing long-span bridge.