This book set out to develop using novel electromagnetic structures an innovative sensor for environmental detection based on a microwave Substrate Integrated Waveguides (SIW) technology. This technology aims to realize microwave passive sensors able to be integrated in wireless sensor network while keeping high performances in terms of sensitivity and selectivity. The proposed SIW environmental sensor has the advantage of providing a new fabrication solution for RF/microwave environmental sensing and greatly simplifies the sensor's manufacturing processes and maintains low costs. For wireless environmental sensor, it is important to associate a tag to each SIW sensing element for wireless communication identification. The book has introduced a wireless communication channel for microwave RFID tags. The tags are based on SIW cavity resonators where each resonator is connected to an antenna. The communication channel is modeled using a two-port matrix model consisting of the resonator and the antennas mathematical representation.