This paper presents an overview of the state-of-the-art and the research trends of substrate integrated waveguide (SIW) technology, which represents an emerging approach for the implementation and integration of microwave and millimeter-wave components and wireless systems. Innovative materials and new fabrication techniques, along with a variety of novel applications, are presented, with particular focus on low-cost and eco-friendly wireless systems for wireless sensor networks and the future Internet of Things (IoT).