While integrated circuits are routinely modified using Focused Ion Beam systems (FIB), the reliability of these modifications has not yet been thoroughly studied. For several years, researchers at Sandia National Labs and CNES have been involved in the evaluation of the impact of FIB exposure on semiconductor structures. We have all come to the same conclusion: the intrinsic behavior of a circuit is altered after FIB intervention and the damage cannot be completely recovered but can be controlled. Despite these results, modified circuits are used in many applications such as satellites or even more critical environments. Although FIB modifications are invasive to the circuit they provide a working sample that can prove out, in silicon, a design change. However, is the functionality of FIB modified ICs reliable? In more practical terms: Can we use modified devices for our applications and what guarantee do we have that they will work after a few months? To answer these questions, we have conducted extended studies addressing both MOS and bipolar circuits. We used basic structures (such as transistors and diodes) and complex structures (operational amplifiers, oscillators, etc) and studied the effects of two different FIB systems, a Schlumberger P2X and an FEI Vectra 986. We have investigated the reliability of the devices by monitoring intrinsic parameters, before FIB, after FIB, during life testing and after life testing.