The development of gray-scale technology, a batch 3-D silicon fabrication technique, has the potential to lift the vertical design restrictions that plague MEMS designers. Gray-scale technology involves two primary steps, beginning with gray-scale lithography, where partial exposure of a photoresist film creates a gradient structure in resist after development. Next, deep reactive ion etching (DRIE), is used to transfer the gradient photoresist structure into an underlying silicon substrate, where the etch selectivity determines the vertical amplification of the gray-scale photoresist structure into a final 3-D silicon structure. Research in our group has focused on investigating the limitations and tradeoffs of the core fabrication technique, as well as incorporating this technology into the design and fabrication of both static and dynamic MEMS devices. We first created an empirical model to relate the height of a photoresist feature to the local transmitted intensity through a projection lithography system. This model enables nearly arbitrary photoresist profiles to be created by simply designating the size of individual sub-resolution pixels on an optical mask. Upon creating a precise 3-D photoresist feature, extensive etch characterization during DRIE was necessary to investigate the effects of various etch parameters on the transfer of gray-scale features into silicon. Etch variables such as silicon loading, electrode power, and oxygen content were studied as they relate to etch selectivity, enabling creation of 3-D silicon structures millimeters wide and up to 100’s of micrometers tall. Static MEMS structures developed during the course of this research have included: (1) a variable span micro-compressor towards increasing cycle performance of a micro-gas turbine generator device (in association with MIT and ARL), (2) 3-D substrate and interconnect technologies for MOSFET relay packaging (in association with Toshiba), and (3) x-ray silicon phase Fresnel lenses with precise profiles for increased focusing efficiency (in association with NASA - Goddard Space Flight Center). Dynamic MEMS devices benefiting from the incorporation of 3-D components have also been demonstrated by our group, such as voltage-tunable MEMS resonators and multi-axis optical fiber actuators for in-package alignment of fibers to edge-coupled optoelectronic components.