FEST3D is a very efficient software tool for the analysis and design of passive RF and microwave components using advanced modal techniques based on waveguide technology, and is the first commercial software capable to integrate high power effects in the design process. FEST3D is able to efficiently analyse different kinds of passive RF structures in waveguide technology. It is based on an integral equation technique efficiently solved by the Method of Moments. FEST3D is able to simulate complex passive devices in extremely short times (of the order of seconds or few minutes), whereas general purpose software (based on segmentation techniques such as finite elements or finite differences) can spend hours for the same calculations.