This paper reports on an attempt to quantify the extent and seriousness of crime and vandalism on urban transit systems. Although many imprecisions in the recording of criminal incidents and the computing of vandalism costs impose limitations on the data, the authors believe that the findings constitute a significant first step toward knowledge of the incidence of transit crime and the monetary costs of transit vandalism. On the basis of data obtained from 37 US transit systems, the total number of criminal incidents on all systems in 1971 is estimated at approximately 33,000 to 39,000. No functional relationships were found between various factors such as total crime indexes and total crime per 100,000 vehicle-miles or 100,000 revenue-passengers. A computed transit exposure index led to the tentative conclusion that the risk of being involved in a criminal incident could be at least twice as great when riding on urban transit vehicles as in nontransit circumstances. If this conclusion is sound, the problem of crime on transit systems may be proportionately more serious than has been generally credited. The total national transit vandalism costs for 1971 are estimated at $7.7 million. Direct transit vandalism costs on the average amounted to less than 0.5 percent of operating costs in 1971, but the problem assumes greater dimensions when indirect costs are also considered. Window breakage was the largest component, followed by damage to seats, damage to stationary facilities, and graffiti. National transit system costs of liability claims resulting from incidents of crime and vandalism in 1971 are estimated at $1.85 million to $2.33 million.