Significant effort in optical-fiber research has been directed in the past few years towards creation of mode-division multiplexing on fiber platforms to further scale the communication bandwidth transmitted per fiber. At the world's leading global conference for optical communications (i.e. Optical Fiber Communication Conference), mode-division multiplexing (MDM) has been one of the hottest topic in the recent years depicted by the large amount of contributed and invited talks in this field. David Richardson et al. [Nature Photonics May 2013] wrote a review letter to discuss the importance of space-division multiplexing in optical fibers to meet the increasing transmission capacity demand. In contrast, current integrated photonics operate almost exclusively in the single-mode regime and typically utilize wavelength-division multiplexing (WDM) alone. MDM is rarely considered to be implemented in integrated photonics due to several challenges. The challenges include creating mode (de)multiplexers with low modal crosstalk and loss and concurrently support WDM (a key feature of many integrated-optics interconnect designs). Here in this dissertation we show the first demonstration of simultaneous mode- and wavelength-division multiplexing with low modal crosstalk and low loss in integrated photonics. Our approach would potentially increase the aggregate data rate for on-chip ultra-high bandwidth communications. We first start off with the discussion of the current status of the data traf- fic demand by the consumers and why there is a need for silicon photonics to meet this demand. We then propose a new silicon waveguide technique to improve the optical loss of silicon waveguides. We make use of this fabrication technique in fabricating high-quality factor microring resonators. We also investigate the nonlinear effects in microring resonators. Acquiring this knowledge about the nonlinear effects in microring resonators, we can engineer the microring resonators design to suit the needs of our system. We utilize adddrop microring filters as the (de)multiplexers in the wavelength-division multiplexing platform. We also introduce an interleaver based on triple-microring integrated with Mach-Zehnder interferometer to separate a comb of closely located channels. The highlight of the dissertation is to discuss how we can implement mode-division multiplexing simultaneously with wavelength-division multiplexing in integrated photonics. Finally we propose a future work for a truly integration of on-chip multiplexing system.