Frequency selective surfaces (FSSs) are periodic arrays of passive structures that exhibit either reflection or transmission response depending on their shielding applications. FSS finds their significant employability in applications, that mostly rely in microwave to terahertz frequency range. Wireless Security and military/defense applications within the limited X-band could potentially utilize FSS to achieve shielding, reduction in Radar Cross Section (RCS) of electrically larger objects and many other applications. Their interesting features can also be used to offer increased performance in antennas. In addition, research into FSS is a continuing endeavor where novel design and analysis of FSS for appropriate applications can be required. The constraints in designing FSS structures for shielding includes miniaturization, wideband shielding effectiveness, incidence angular stability and bendable/conformal configurations. Due to ever increasing demand of high data rate and larger bandwidth, wireless communication started relying on using combination of Ultra-Wideband (UWB) and Multiple Input Multiple Output (MIMO) technology. However, sufficient shielding/isolation and compactness is required for UWB-MIMO systems. These requirements become more prominent when non-planar UWB-MIMO iv systems are designed for 3D system-in-package applications. Likewise, modern communication services also require wideband and enhanced performance antennas incorporated in high gain communication devices. The reported methods only offer performance enhancement for a limited band while sacrificing overall compactness. Miniaturization and wideband performance enhancement is still challenging to achieve for antennas incorporated in high gain communication devices. This research work presents design and analysis of novel planar and bendable/conformal FSS configurations, the proposed conformal configurations include outward, inward and double curved FSS shields on flexible laminates. Miniaturized, wideband and highly selective FSSs are also proposed for both planar and conformal configurations, that meet all the design constraints. The shielding performance is analyzed and validated by simulations, analytical modeling, full wave FE-BI (Finite Element-Boundary Integral) analysis, fabricated system and measurements. In this research, wideband and efficiently shielded non-planar UWB-MIMO systems are presented for 3D system-in-package applications by employing FSSs. Furthermore, a novel FSS shield is investigated to propose a miniaturized, wideband and high gain antenna-FSS arrangement for high gain communication devices. This antenna-FSS arrangement is compact, less complex while offering enhanced wideband performance. Keywords: Frequency Selective Surfaces (FSS), Electromagnetic Shielding, Shielding Effectiveness (SE), Radomes, Ultra-wideband (UWB), System-in-Package, High impedance surface (HIS), Gain Enhancement, Electronic band gap (EBG), Metamaterials, Artificial magnetic conductor (AMC), Partial reflecting surface (PRS)