Several conventional and advanced mapping techniques are frequently utilized to accomplish a successful catheter ablation. Many of these mapping techniques (activation mapping, entrainment mapping) are hampered by either hemodynamic instability of some tachycardias or non-sustainability of others. Pacemapping is a commonly used tool for mapping non-sustained or hemodynamically unstable VT, which is based upon the principle that activation of the heart from a given site will yield a reproducible body surface electrocardiogram (ECG) morphology and that pacing from a site very close to the site at which VT activates the heart will result in a matching ECG morphology. This technique, however, is limited by imperfect accuracy and spatial resolution, subjectivity of interpretation leading to marked inter-observer variability in the perceived quality of a morphologic match, and by the need for an intuitive interpretation of the ECG to direct catheter manipulation. We hypothesize that one can improve the accuracy with which the origin of VT is localized by applying body surface potential mapping (BSPM), using data derived from 120 simultaneously acquired ECGs.