This thesis reports on a new low-cost wafer-level packaging technology for microelectromechanical systems (MEMS). The MEMS process is based on a revised version of High Aspect Ratio Polysilicon and Single Crystal Silicon (HARPSS) technology. The packaging technique is based on thermal decomposition of a sacrificial polymer through a polymer overcoat followed by metal coating to create resizable MEMS packages. The sacrificial polymer is created on top of the active component including beams, seismic mass, and electrodes by photodefining, dispensing, etching, or molding. The low loss polymer overcoat is patterned by photodefinition to provide access to the bond pads. The sacrificial polymer decomposes at temperatures around 200-280aC and the volatile products permeate through the overcoat polymer leaving an embedded air-cavity. For MEMS devices that do not need hermetic packaging, the encapsulated device can then be handled and packaged like an integrated circuit. For devices that are sensitive to humidity or need vacuum environment, hermiticity is obtained by deposition and patterning thin-film metals such as aluminum, chromium, copper, or gold. To demonstrate the potential of this technology, different types of capacitive MEMS devices have been designed, fabricated, packaged, and characterized. These includes beam resonators, RF tunable capacitors, accelerometers, and gyroscopes. The MEMS design includes mechanical, thermal, and electromagnetic analysis. The device performance, before and after packaging is compared and the correlation to the model is presented. The following is a summary of the main contributions of this work to the extensive research focused on MEMS and their packaging: 1)A new low-cost wafer-level packaging method for bulk or surface micromachined devices including resonators, RF passives and mechanical sensors is reported. This technique utilizes thermal decomposition of a sacrificial polymer through an overcoat polymer to create buried channels on top of the resonant/movable parts of the micromachined device. It provides small interconnections together with resizable package dimensions. We report MEMS package thicknesses in the range of 10 mm to 1 mm, and package size from 0.0001 mm to 1 mm. 2)A revised version of the HARPSS technology is presented to implement high aspect ratio silicon capacitors, resonators and inertial sensors in the smallest area.