In this eSeminar we will review the fundamental aspects of RF Breakdown in gasses. In particular, we will discuss the main parameters affecting the discharge breakdown threshold such as the frequency of operation, the pressure, the temperature or the device dimensions. Simulations with SPARK3D® coupled with CST Studio Suite® will show the benefits of using a full numerical approach to determine the breakdown power level in comparison with analytical approaches. For the particular case of narrowband bandpass filters we will show the advantages of using SPARK3D combined with Filter Designer 3D and CST Studio Suite to determine the breakdown power level in a filter cavity without the need to design the complete microwave filter.