Cellulose ethers are the most commonly used materials for the design and fabrication of oral modified-release dosage forms. This family of polymers includes hypromellose, hydroxypropyl cellulose, hydroxyethyl cellulose, sodium carboxymethyl cellulose, and ethyl cellulose (EC). The cellulose ethers are widely used primarily due to their long history of safe and effective use in the development and manufacture of modified-release technologies. Indeed, as early as 1968 a detailed account of drug release mechanisms from a hypromellose-based matrix tablet was reported (1). However, despite decades of use in what are now fairly standard applications, such as drug polymer matrix systems, innovations are required to produce highly functional materials and polymer systems to meet the demands for improved performance and reliability of current technologies in addition to developing the next generation of advanced modified-release technologies. In our laboratories we have taken a rational approach to advancing cellulose-based materials and technologies through fundamental structure-function studies. In this monograph we highlight two such examples viz., highly compressible, high crystallinity EC, Aqualon T10 EC for enhanced matrix diffusion control, especially in compressed devices with complex geometries and modified-release grades of hydroxypropyl cellulose, Klucel hydroxypropyl cellulose, that permits precise targeting of erosion rates and/or diffusion versus erosion mechanisms based purely on molecular weight.