The wide deployment of Dense Wavelength Division Multiplexing (DWDM) optical network is inevitable with ever expanding internet and wireless traffic. Conventional optical network components, such as discrete switch, and variable attenuator, are becoming increasingly unattractive, expensive, and very often, infeasible to be used for the implementation of a communication system with such high channel density. For years, MicroElectroMechanical Systems (MEMS) technology has been a critical player for many discrete network functions with unmatched performance on chromatic dispersion, insertion loss, speed and reliability. An often over-looked character of MEMS is its large scale integratibility, which provides a unique potential solution for developing building blocks of fundamental DWDM functions. Several important MEMS applications in DWDM will be reviewed with emphasis on critical functions such as wavelength switching, wavelength interleaver, multi-channel add, drop, multiplexing and demultiplexing.