This chapter reviews concepts related to the uncertainties associated with structures and how the results of load tests can be used to reduce these uncertainties. When an existing bridge is subjected to a load test, it is known that the capacity of the cross section is at least equal to the largest load effect that was successfully resisted. As such, the probability density function of the capacity can be truncated after the load test, and the reliability index can be recalculated. These concepts can be applied to determine the required target load for a proof load test to demonstrate that a structure fulfills a certain reliability index. Whereas the available methods focus on member strength and the evaluation of isolated members, a more appropriate approach for structures would be to consider the complete structure in this reliability-based approach. For this purpose, concepts of systems reliability are introduced. It is also interesting to place load testing decisions within the entire life cycle of a structure. A cost-optimization analysis can be used to determine the optimum time in the life cycle of the structure to carry out a load test.@en