Nothing comes without costs. Electromagnetic interference (EMI) has been one of the first costs humans started identifying after initial achievements on how electromagnetic waves interact with different substances. Shielding against EMI emissions became one of the prominent challenges for the Electromagnetic Compatibility (EMC) community. The extent of the benefits of EMI shielding ranges from healthcare applications to commercial/industrial ones. The dissertation aims to solve two major shielding problems in the context of passive and active methods. The first problem is the nearfield shielding characterization of Frequency Selective Surfaces (FSSs), where the near-field performance of FSS structures is analyzed and differentiated from far-field performance. After introducing sufficient analytical models and techniques to characterize shields' near-field responses, an analysis is provided to reveal the fundamental failure of conventional FSS-based shields in providing NF shielding. Then, a new flexible bandstop FSS is introduced that provides stable near-field (NF) characteristics in the X-band. Thorough analyses are carried out to monitor the effects of incident angle, edge diffraction, bending, and wave polarization on shielding. The dissertation investigates further the active techniques that enable the user to harness the incident signal while providing the required shielding. This part of the study investigates reconfigurable intelligent surfaces for shielding or amplification, where a space-time-modulated (STM) medium is designed as a potential solution for green shielding purposes. Then, the green shielding possibilities are investigated through four different mechanisms. It turns out that STM media can be influential for other similar purposes, encouraging future attempts to realize the media.