This Thesis is devoted to the study of subwavelength grating (SWG) structures: extremely versatile optical metamaterials that have become a fundamental tool for the design of ultra high-performance photonic devices. The importance of SWG structures arises from their capability to synthesize artificial materials with tailorable optical properties, including refractive index, dispersion or anisotropy. A complete framework for SWG metamaterials is provided along the Thesis, including a revision of the different procedures and electromagnetic tools used along this Thesis to demonstrate proposed SWG topologies and devices, a theoretical study of wave propagation through SWG structures and the models predicting their homogeneous behavior, and the proposal, design and experimental characterization of various SWG topologies and devices.