This paper analyzes 21 years of data related to unintentional hazardous materials (hazmat) releases on air, marine, and rail transportation modes reported in the United States (U.S.) -- although their origins and destinations may be outside the U.S. The authors' analysis reveals thousands of cases have occurred and their impacts vary by mode. These impacts include material losses, carrier damages, property damages, response costs, and remediation and clean-up costs. There appears to be some reduction in the frequency of incidents and accidents as regulations have been promulgated, although the authors cannot attribute causation. They review suggests that enhanced regulations and attentiveness have probably led to better reporting of hazmat occurrences. Moreover, developing and maintaining safer processes and designing safer products, containers, and systems can play an important role in minimizing hazmat releases.