The potential of microelectromechanical systems (MEMSmicroelectromechanical system (MEMS))/nanoelectromechanical systems (NEMSnanoelectromechanical system (NEMS)) technologies has been viewed as a revolution comparable to or even greater than that of microelectronics. The scientific and engineering advancements in MEMS/NEMS could enable applications that were previously unthinkable, from space systems, environmental instruments, to appliances for use in daily life. As presented in previous chapters, development of core MEMS/NEMS processes has already demonstrated many commercial applications as well as potential for advanced functionality in the future. However, low-cost and reliable packaging for protection of these MEMS/NEMS products remains a very difficult challenge. Without addressing these packaging and reliability issues, no commercial products can be sold on the market. Packaging design and modeling, packaging material selection, packaging process integration, and packaging cost are the main issues to be considered when developing a new MEMS packaging process. In this chapter, we present the fundamentals of MEMS/NEMS packaging technology, including packaging processes, hermetic and vacuum encapsulation, wafer-level packaging, three-dimensional (3-Dthree-dimensional (3-D)) packaging, polymer-MEMS assembly and encapsulation, thermal issues, packaging reliability, and future packaging trends. Specifically, development of MEMS packaging will rely on successful implementation of several unique techniques, including packaging design kits for system and circuit designers, low-cost and high-yield wafer-level, chip-scale packaging techniques, effective testing techniques at wafer level to reduce overall testing costs, and reliable fabrication of an interposer [43.1] with vertical through interconnects for device integration.