The properties of glass optical fibers are not significantly different from those found in a form of bulk or sheet glass. A notable exception with smooth glass fibers is that the strength degrades quickly with small surface cracks and flaws that can result from handling and material processing. The strength of glass optical fibers is governed by mechanical properties similar to those of crystalline solids. Glass is elastic up to the fracture point and will fail in tension before it does in compression. Glass is considered a brittle material because no plastic flow or permanent deformation takes place when tensile fracture occurs. Since glass is a strong elastic material it will obey Hooke's Law over a wide range of stresses, therefore the stress in an optical fiber only depends upon two elastic constants: Young's modulus, E, and the bending strain, ε. Under bending the cross sectional area will also neck down and a decrease by the factor known as Poisson's Ratio6, μ. The Poisson effect is important in calculating stress, but is usually ignored by the fiberoptic industry. This paper addresses the use of Poisson's Ratio in the stress equations for both bending and twisting strains.