Body surface mapping records the electrocardiogram (ECG) on the entire body (torso) surface using a grid of many leads (more than 30) on the torso surface. Although many studies have demonstrated that mapping provides more information than the standard 12-lead ECG, it is still only a powerful scientific tool but not in widespread clinical use. The challenge is to retrieve and display diagnostic information such that it can be assessed by the naked eye. This chapter first provides an overview on basic tools and standards for ECG mapping. Here, more careful data preprocessing and better instrumentation is required than for the 12-lead ECG. Mathematical tools for estimating the number of independent signals are discussed. The classical potential and parameter maps are addressed shortly. Recently, the model-based back projection of the body surface ECG data onto the cardiac surface (inverse problem, functional imaging) has been successfully validated in in vitro models and humans. A short introduction to the basics of functional imaging is provided and two promising approaches are highlighted. Limitations and potential for further improvement are discussed. Thus, the additional information in many leads needs sophisticated methods for displaying the source pattern on the actual cardiac geometry. The sole improvement of instrumentation in terms of more leads without the support of tailored algorithms does not provide any new diagnostic information.