The ability to selectively redirect specific wavelengths of light has attracted a lot attention for photonic crystal materials. Presently, there is a wealth of research relating to the fabrication and application of photonic crystal materials. There are a number of structures that fall into the category of a photonic crystal; 1D, 2D, and 3D ordered structures can qualify as a photonic crystal, provided there exists ordered repeating lattices of dielectric material with a sufficient refractive index contrast. The optical responses of these structures, namely the associated photonic bandgap or stopband, are of particular interest for any application involving light. The sensitivity of the photonic bandgap to changes in lattice size or refractive index composition creates the possibility for accurate optical sensors. Optical phenomena involving reduced group velocity at wavelengths on the edge of the photonic bandgap are commonly exploited for photocatalytic applications. The inherent reflectivity of the photonic bandgap has created applications in optical waveguides or as solar cell reflector layers. There are countless examples of research attempting to exploit these facets of photonic crystal behavior for improved material design. Here, the role of photonic crystals is reviewed across a wide variety of disciplines; cataloging the ways in which these structures have enhanced specific applications. Particular emphasis is placed on providing an understanding of the specific function of the tunable optical response in photonic crystals in relation to their application.