This chapter includes a review of MEMS resonators for use as frequency references and filters. Basic modeling and transduction techniques for resonators are described with emphasis on capacitive transduction. A brief history of resonator architectures is given, beginning with early devices consisting of linear comb drive actuators and flexural beams to later devices utilizing bulk resonant modes and parallel plate actuation. The underlying causes of frequency instability are described, with emphasis on the dependence of frequency upon temperature. Descriptions are provided for the various energy dissipation mechanisms that are central to determining the quality factors of resonators. The concept of coupled resonators for narrow-band frequency filters is also covered.