Micro Electro Mechanical Systems (MEMS) are popular alternatives to complex electronic circuits in RF systems due to their better performance, ease of fabrication and reliability. Many of these devices operate in their resonant mode and hence are limited by a fixed work frequency. This book presents parametrically excited MEMS oscillators whose working frequency can be varied by an external pilot signal, using frequency locking or entrainment phenomenon. A nonlinear dynamic model is presented, which is verified by numerical simulations. Details of the model are studied analytically using perturbation methods, revealing bifurcations in the system. As these devices consist of actively vibrating components, a large portion of their energy is lost to the substrate via anchor. Very few methods exist to prevent these losses, which become predominant at higher frequencies.Adapting ideas from building isolation techniques a design called MESA is presented, which reflects energy that was lost to the substrate in the form of Surface Acoustic Waves(SAW). Optimal design for MESA is investigated in the parameter space using Finite Element Analysis and experiments.