The Grating Light ValveTM (GLVtm) spatial light modulator is a unique and proven CMOS process-compatible optical MEMS device. The modulator employs a dynamically adjustable diffraction grating to manipulate an optical signal. Today, the GLVTM technology is successfully used in high-resolution display and imaging systems, where its high efficiency, large dynamic range, precise analog attenuation, fast switching speed, high reliability, high yield, and the ability to integrate thousands of channels into a single device are fundamental advantages. These same properties make the GLVTM device desirable for optical telecommunication applications. The optical properties, functionality, device design, and CMOS processing of the GLVTM will be presented. Challenges and solutions that arise from adapting the current GLVTM technology to optical telecommunications wavelengths will be discussed. Measured results will be presented that describe GLVTM performance parameters, including insertion loss, dynamic range, polarization dependent loss, and spectral attenuation accuracy.