Microelectromechanical systems (MEMS) are an essential ingredient in many technological innovations and a source of game-changing inventions in the automotive industry, space exploration, consumer electronics and medical applications due to its capability to control mechanical effects on a micrometer scale. Especially where accessibility, connectivity, system size and energy supply are limited, microsystems can be an enabling technology for sensing, actuation and data transmission. However, their applicability depends on their ability to operate reliably even in extremely hazardous situations such as in space, where the devices are exposed to high levels of radiation, large temperature variations and accelerations. Improving the design and increasing the lifetime of devices requires the identification and detailed understanding of failure modes. This thesis was aimed at deepening the understanding of fabrication-related effects and space-relevant environmental hazards on the mechanical properties of MEMS on the material and the systems level. Although countless microsystems have been developed in recent years, several vital questions related to the reliability of MEMS systems and MEMS materials remain open to date. Two topics which merit special attention were at the center of this study: First, the influence of packaging on the functioning and the reliability of microsystems and second, the reliability of microsystems and their materials under the harsh environmental conditions imposed by space applications. Out of this area of interest the following questions were studied: Strain analysis in MEMS package by HRXRD: Packaging offers protection to the microsystem and aids to maintain a stable environment. However, it also influences the strain distribution in the system, and the energy losses in resonant structures due to air being squeezed in thin gaps between the resonator and the package. The distribution of residual stresses and bonding stresses in a MEMS wafer-level package was analyzed by high-resolution x-ray diffraction (HRXRD) and the possibility of nondestructive investigation of the strain distribution in sub-surface structures was demonstrated. Air damping in MEMS packages: An improved framework for the analysis of air-damping in microresonators and their packages has been presented. Understanding the pressure sensitivity allows determining the admissible pressure levels in resonator packages, above which the performance is deteriorated by losses through air-damping. In addition, monitoring the resonance frequency and quality factor variations over time can be used to determine the hermeticity and leak rates in very small packages where the traditional helium leak test method fails because the relevant leak rates are below its sensitivity. Reliability of MEMS under space-relevant environmental hazards: The performance of resonant microstructures is directly linked to the Young’s modulus of the utilized material and therefore they are very sensitive to variations thereof. The susceptibility of microfabricated structural materials (SU-8 and silicon) to proton irradiation was investigated. In order to isolate the influence of radiation on the materials themselves, single material resonators with contactless actuation and readout were tested. The experimental results showed that single crystal silicon and SU-8 were tolerant to high doses of proton radiation and are hence very well suited for MEMS in space applications.