In nanoscale or mesoscopic regimes, the traditional models for the descriptions of electromagnetic and optic phenomena are not directly applicable. The boundary conditions of the system play important role in determining the manifestations of the corresponding physical phenomena. Many familiar concepts and simulation methods for dealing with the problems of light, acoustic and quantum waves under this new situation should be reconsidered and modified in order to yield correct results. Here mesoscopic system refers to the system that has characteristic lengths in the same order of scale as the wavelengths of the corresponding wave phenomena we are interested in. Usually the devices working in this regime have properties different from that of the traditional devices, thus we cannot rely on the old knowledge, and should study them theoretically and numerically before manufacturing them in order to avoid unnecessary trial-and-error processes and the waste of time and money. Theoretical Optics Research serves for solving problems of the kind mentioned above. Specifically, we design and investigate the properties and utilities of various nano- and meso-structures for manipulating light, electromagnetic excitations, and acoustics/elastic waves. The phenomena and devices we deal with include photonic and sonic crystals, metamaterials and transformation optics, physics properties of nanomaterials, the working principles of nanodevices, and applications of these devices.