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In this review, we summarize and discuss our recent studies of subwavelength grating (SWG) structures for engineering the refractive index of silicon microphotonic waveguides. The SWG effect allows control of the effective refractive index of a waveguide core over a range spanning the values of the cladding material and silicon by lithographic patterning. We demonstrate this effect with the example of segmented photonic wire waveguides, which are shown to exhibit low propagation loss, and can be used to make highly efficient waveguide crossings and in-plane fiber–chip coupling structures. Other applications of SWG structures in silicon photonic waveguide devices include surface grating couplers with enhanced performance and simplified fabrication requirements, as well as a novel curved waveguide sidewall grating microspectrometer, in which an SWG structure fulfills a dual purpose by acting as an effective slab waveguide for diffracted light and as a lateral cladding for a channel waveguide.