Methodologies using bridge dynamic responses or modal information for system identification are very attractive and highly desirable. In this book, identification methodologies for bridge-vehicle systems using such information are developed. A bridge model updating method is developed using the natural frequencies and static deflections or strains of the bridge. Methodologies of identifying dynamic vehicle axle loads and vehicle parameters are also developed using only the field measured time histories of bridge responses. Dynamic impact factor for multi-girder concrete bridges is also studied and statistical properties are obtained. Field tests are conducted and results have demonstrated the ability of the developed methodologies to identify the parameters of bridge model, dynamic vehicle axle loads, and vehicle parameters with satisfactory accuracy. The developed methodologies in this book can be used by bridge researchers and engineers to update bridge models and to improve the existing bridge weigh-in-motion techniques. The proposed expressions for impact factor can be used as modifications to current design codes.