This paper presents the approach and methodology of the Large Truck Crash Causation Study (LTCCS), undertaken jointly by the Federal Motor Carrier Safety Administration and the National Highway Traffic Safety Administration. The LTCCS is a study of a nationally-representative sample of serious and fatal heavy truck crashes occurring between 2001 and 2003. The data collected provides a detailed description of the physical events of the crash, along with an unprecedented amount of information about the vehicles, drivers, truck operators, and environment. The LTCCS was designed to include all elements in a traffic crashvehicle, driver, and environment. In addition, extensive information is collected about the operator of each truck involved, including details about driver compensation, vehicle maintenance, and carrier operations. Rather than crash experts assigning causes to each crash, the LTCCS approach is based on statistical associations in the aggregate data. The crash assessment data provides information on what physically happened in the crash, including prior movements of each vehicle, the critical event in the crash, and the reason for the critical event. Causes can be determined through the analysis of this information, by identifying associations between vehicle, driver, and environmental characteristics, and particular crash types or modes of involvement. The approach of the LTCCS is consistent with the probabilistic nature of traffic crashes. Analysis of the data proceeds by searching for associations between the various descriptive variables and involvements in particular types of crashes. The broad range of factors included permits a wide range of hypotheses to be tested.