Nonpolar and semipolar III–nitride quantum wells (QWs) and devices have been extensively studied due to their unique valence band (VB) structure and polarized optical emission. Unlike conventional c-plane oriented III–nitride QWs, the low crystal symmetry and unbalanced biaxial stress in nonpolar and semipolar QWs separates the topmost VBs and gives rise to polarized optical emission. Since the first experimental reports on nonpolar devices, research on this topic has progressed very rapidly and has covered nonpolar m-plane and a-plane QWs and devices as well as semipolar , , and QWs and devices. Issues such as strain, plane inclination angle (with respect to the c-plane), indium composition, temperature, and their impact on QW VB structure and device performance have been extensively studied. In this paper we review the physical background and theoretical analysis of the VB states and polarized optical emission of nonpolar and semipolar structures and discuss their potential impacts on optoelectronic devices. Experimental results for nonpolar and semipolar light-emitting diodes and laser diodes will be covered along with additional discussions on the potential applications and challenges related to their unique physical properties.