To be able to accurately design structures and make reliability predictions in any field, it is first necessary to know the mechanical properties mechanical property nanostructure mechanical property of the materials that make up the structural components. The devices encountered in the fields of microelectromechanical systems (MEMS) MEMS and nanoelectromechanical systems (NEMS), NEMS stress strength Young’s modulus are necessarily very small, and so the processing techniques and the microstructures of the materials used in these devices may differ significantly from bulk structures. Also, the surface-area-to-volume ratios in such structures are much higher than in bulk samples, and so surface properties become much more important. In short, it cannot be assumed that the mechanical properties measured for a bulk specimen of a material will apply when the same material is used in MEMS and NEMS. This chapter will review the techniques that have been used to determine the mechanical properties of micromachined structures, especially residual stress, strength and Young’s modulus. The experimental measurements that have been performed will then be summarized, in particular the values obtained for polycrystalline silicon (polysilicon).