This chapter presents principles and justification of diagnostic load tests of bridges as normally performed. Normally, diagnostic tests serve to verify and adjust the predictions of an analytical model. However, as presented in the chapter, the results of a diagnostic load test in a bridge can also serve other objectives. Several examples of application are presented with the main objective not only to show how the tests are carried out, but chiefly to introduce which are the main issues to take into account to obtain accurate and reliable results that can be used in the assessment of the actual capacity of the bridge. In the case of static tests, conclusions regarding the measurement stabilization time are presented. For dynamic tests in railway bridges, the extrapolation to lower and higher speeds is also discussed.