In 2000 the Deutscher Ausschuss für Stahlbeton (DAfStb) published its guideline for load testing of existing concrete structures and therewith the experimental proof of the load-bearing capacity became a widely accepted method in Germany. Since then over 2000 proof load tests have been carried out, and so a lot of experience exists in the usage of the Guideline. Nevertheless, there are still reports about load tests carried out with mass loads or using mechanical measurement techniques. To prevent a misuse of the Guideline in future, to take into account the great experiences existing in the evaluation of the measuring results and the bearing condition, the DAfStb decided to revise the existing Guideline. In recent years there has also been an increased research activity focusing the development of evaluation criteria for possible brittle failures in shear and to extend the safety concept for the evaluation of elements not directly tested. These results will also be considered in the new Guideline. This chapter gives an overview of the rules of the existing Guideline and discusses these in the background of the gained experience during the last 17 years. The new criteria for evaluating the shear capacity are explained, and the thoughts of the new safety concept for load testing are introduced.