I will go over the basic interactions between the MRI system and the implantable medical devices focusing on the RF heating. MRI has three kinds of fields: static field B0, gradient fields, and radiofrequency (RF) fields. Individual field or combined fields can interact with the implanted devices and might cause tissue damage from heating, unwanted nerve stimulation, and image distortion. The RF energy from MRI can cause tissue heating around the implantable devices, especially around long metallic structures such as pacemaker leads or deep brain stimulator leads. It might also be rectified by the device and cause unwanted stimulation of the tissue surrounding the electrodes. The gradient fields produce eddy currents on the surface of the devices and might cause devices and tissue heating. The gradient fields can also cause unwanted stimulation at the electrode-tissue interface since their frequency range is comparable to the therapeutic frequency. Combined with the static field, they can also generate mechanic vibration of the device that might cause tissue damage or patient’s discomfort. The greatest challenge of making MRI safe devices is to accurately characterize all the interactions between MRI fields, the human bodies, and the medical devices. Computer modeling has been an indispensable tool to tackle this issue. Challenges in RF and thermal modeling will be discussed.