Micro-Electro Mechanical Systems (MEMS) are a fast growing field in microelectronics. MEMS are commonly used as actuators, and sensors with a wide variety of applications in health care, automotives, and the military. The MEMS production cycle can be classified as three basic steps: 1. design process, 2. manufacturing process; and 3. operating cycle. Several studies have been developed for steps 1 and 2, however, information regarding criticality analysis of operational failure modes in MEMS is lacking, and thus, the application of reliability engineering methodologies is needed. MEMS are extremely diverse, and failure modes can be unique for each device. In this study, a conceptual framework for the assessment of the criticality of key failure modes in MEMS accelerometers is proposed. The conceptual framework establishes seven steps to perform the criticality analysis. The first step consists in the selection of the particular MEMS device and associated technical specifications. The second considers the key environmental conditions for the device's operation. The third and fourth are the selection of the failure mechanism class, and the definition of the failure mechanisms under the given environmental conditions. The fifth step deals with determining the device's common failure modes. Steps six and seven involve the development and implementation of the Failure Mode, Effect and Criticality Analysis (FMECA). Thirteen MEMS failure modes were analyzed under three different scenarios, and the obtained results discussed. The conceptual framework was successfully completed, the results were validated, and the effectiveness of the applicability of FMECA to automotive MEMS established.