The spectral and directional control of radiative properties by utilizing engineered micro/nanostructures has enormous applications in photonics, microelectronics, and energy conversion systems. The present dissertation aims at: (1) design and analysis of micro/nanostructures based on wave interference and magnetic resonance effects to achieve tunable coherent thermal emission or enhanced optical transmission; (2) microfabrication of the designed structures; and (3) development of a high-temperature emissometer to experimental demonstrate coherent thermal emission from fabricated samples at temperatures from 300 K to 800 K. Asymmetric Fabry-Perot resonant cavities were studied as a potential coherent emission source. The reflectance was measured at room temperature using a Fourier-transform infrared spectrometer, and the emittance can be indirectly obtained from Kirchhoff's law. A high-temperature emissometer was built to measure the thermal emission of fabricated samples, and the temperature effect on the emission peaks was discussed. The direct and indirect approaches were unified and a generalized Kirchhoff's law was deduced to calculate thermal emission from layered structures with nonuniform temperatures. Magnetic polaritons were identified as a mechanism for achieving extraordinary optical transmission/absorption, through the comparison between equivalent capacitor-inductor models and the rigorous coupled-wave analysis. With carefully tuned geometric parameters, the resonance frequencies can be tailored for specific applications. A coherent emission source was designed with grating structures by excitation of magnetic polaritons, and is well suitable for thermophotovoltaic applications, thanks to the spectral selectivity and directional insensitivity of magnetic polaritons. Test samples were fabricated, and coherent thermal emission was experimentally observed at room temperatures up to 800 K. The results obtained in this dissertation will facilitate the design and application of micro/nanostructures in energy-harvesting systems.