**4. Conclusions**

In this chapter, we provide guidelines for the design of novel SIW filters based on the alternation of SIW sections with reduced (perforated) or increased (periodically loaded with metallic cylinders) effective permittivity with ordinary SIW sections, making use of the impedance inverter model. Examples of several SIW filter configurations show that this solution can lead to the design of compact filters with good performance and low sensitivity to fabrication inaccuracies.
