Optical biosensors make up a valuable toolkit for label-free biosensing. This thesis presents a detailed study on resonant grating surfaces for biosensing. The focus is on silicon nitride gratings, which exhibit a guided-mode resonance that is highly sensitive to refractive index variations in the vicinity of the grating. A sensitivity of 143 nm/RIU (refractive index units) is measured, leading to a detection limit of 2.4×10−4 RIU. This performance is shown to be sufficient for the detection of biomolecular binding down to ng/mL concentrations. With out-of-plane excitation, these gratings can be used as a sensing surface, enabling a spatially-resolved measurement of variations in refractive index; resonance imaging. The minimum detection distance (sensing depth) is measured to be 183 nm away from the grat- ing, while the spatial resolution of resonance imaging is found to be asymmetric: 2 μm parallel to, or 6 μm perpendicular to the gratings. Using a novel approach of fabricating a resolution test pattern on top of the grating, the relationship between resolution and index contrast is studied - an important question in the context of biosensing - where it is found to decrease with index contrast. All experimental results are supplemented with theoretical and computational models. The resonant gratings are then extensively applied to the study of biofilm development, cellular imaging, and the imaging of cellular secretion. Finally, a miniaturised biosensor is demonstrated, based on a chirped resonant grating. By tuning the resonance wavelength spatially on the chip, the resonance information is directly translated into spatial informa- tion. Instrument read-out requires just a monochromatic light source and a simple CCD camera, resulting in a final device that is inexpensive, compact, robust and can be remotely operated. Performance is proven with successful detection of biomolecular binding.