We investigate noise and fluctuations in Micro-Electro- Mechanical-Systems (MEMS). MEMS operate on many different subsystem levels and in different energy domains. In the following we analyze two subsystems in more detail: (1) the electronic subsystem represented by the circuitry part of the device, (2) the mechanical subsystem consisting of fragile vibrating mechanical structures. Mode amplitudes of vibrating structures are subject to interaction with their environment, i.e. gaseous, liquid, and electronic systems, each of which shows fluctuations and noisy behavior by itself. Our approach focuses on finding the time-scales and the dominant process in order to determine the noise behavior. By applying the fluctuation-dissipation theorem we are able to extract various response coefficients, such as e.g. the carrier electric and thermal conductivity and the quality factors of the vibrational modes. The impact of response coefficients due to the cross correlations of subsystems may be analyzed. The analysis is performed by numerical simulations of an appropriate model for the different sub-systems in terms of stochastic differential equations of motion for the respective observable quantities. These are the vibrational amplitudes, the electronic densities and the currents. The resulting correlation functions are analyzed.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.