The topic of this chapter is the validation of electrocardiographic inverse problems. We describe three different approaches to validation: purely computational methods, physical and animal experiments, and validation from measurements on humans. Computation methods are typically the easiest to control and the most straightforward to implement and use, but they are limited in their fidelity to real conditions. Physical and experimental models have the richest history and provide somewhat more limited control but approach reality through the use of torso-shaped electrolytic tanks, animal hearts, and even completely instrumented whole-animal preparations. Validation based on human measurements are very difficult to control and present many measurement challenges. However, they represent the most realistic conditions and thus have great relevance. All three approaches are on common use and development today and have a role in rigorous validation schemes. We conclude with a description of some of the remaining challenges to validation as well as some recent developments that suggest imminent significant advances.