Electric and magnetic signals originating outside of a defibrillator may affect its operation, a phenomenon known as electromagnetic interference (EMI). EMI usually refers to interference from environmental electromagnetic instrumentation and should be distinguished from other sources of electromagnetic noise causing ICD malfunction (Table 1). Signals most likely to penetrate and affect pacemakers and ICDs are electromagnetic waves or signals at a frequency or repeat rate falling within the radiofrequency (RF) spectrum (Table 2). EMI in daily life is therefore usually a radiofrequency interference (RFI). Radiofrequency interference tends to occur when signals have carrier frequencies or pulse repetition rates falling in the physiologic range and when the source of emission is very close to the implanted generator. These conditions may for instance occur with cellular (digital) telephones or electronic antitheft surveillance devices (EAS. Although instruments emitting radiofrequency energy have become increasingly abundant in the modern environment, the risk of EMI appears low (1). Special engineering features such as shielding of the circuits in hermetic metal cases, filtering, interference rejection circuits, and avoidance of far field -sensitive “unipolar” electrode systems have succeeded in making pacemakers and ICDs relatively invulnerable to extraneous RF sources (2).