Micro-electro mechanical systems (MEMS) have been used for versatile components as sensors and are called microsystems. This technology is based on advanced arts of microfabrication developed for integrated circuit on a Si (silicon) wafer. The MEMS that have versatile components and circuits on a chip play important roles in advanced systems for user interface, wireless communication, Internet of things, and so on. The surface-micromachined poly Si microstructures were integrated with complementary metal oxide semiconductor (CMOS) circuit as well, being called modular integration of CMOS with microstructures. The advantage of the wafer transfer method is that the structural layer or MEMS are not fabricated on the large-scale integration wafer but on the carrier wafer, and hence the fabrication process has flexibility. Packaging is needed for MEMS and especially a wafer level packaging plays important roles in MEMS. The chapter also presents an overview on the key concepts discussed in this book.