Zinc oxide (ZnO)-based optoelectronics has emerged as a frontier area in semiconductor research in recent years. In the design of ZnO-based optoelectronic devices, cation-substituted ZnO serves as essential components for the desired device functions. Band-gap engineering by cation substitution enables the facile preparation of barrier layers and quantum wells in device structures. Wurtzite solid solutions Zn1-xMgxO, Zn1-xCdxO, and Zn1-xBexO have been reported as examples where band gaps are gradually modulated as functions of x. In this contribution, we present an overview of composition-dependent band-gap variations of Zn1-xMxO solid solutions. In addition, we describe the optical properties and microstructural evolution in polycrystalline Zn1-xMgxO (0 <= x <= 0.15). It is proposed that chemical strain associated with cation substitution has an impact on the band-gap magnitude, crystallite morphology, and optical defects.