The wide and ultrawide direct energy bandgap semiconductors Gallium Nitride (GaN) and Aluminum Nitride (AlN) have revolutionized 21st-century photonics and electronics. Breakthrough inventions such as the (In,Ga)N based blue light-emitting diode (LED) and laser diode (LD) have transmorphed the lighting industry, replacing the incandescent light bulb with the much more efficient red-green-blue (RGB) based white solid state lighting. Along with their (ultra)wide bandgaps, exploitation of the pyroelectric nature and high breakdown field of wurtzite (Al,Ga)N is enabling high mobility electron and hole channel transistors suitable for radio frequency (RF) and high-power devices currently used for high-speed data transmitters, power amplifiers, and CMOS. These two exemplary advances prove these semiconductors are suitable for efficient energy usage and a sustainable future. Although AlN and GaN have already left their footprint in today's information age, the design space of (opto)electronic devices has not been explored completely and many device properties and functionalities can be improved upon by the optimization of crystal growth, the careful study of transport in advanced heterostructures, and heterogeneous integration. Furthermore, a recent resurgence in materials exploration has led to the emergence of 'New Nitrides', which are semiconductors or metals with properties ranging from ferroelectricity, to ferrimagnetism, and superconductivity. This expansion enriches the already extensive toolkit offered by conventional group III/N materials. Yet, the fundamental optical properties and optoelectronic device applications of these 'New Nitrides' have not yet been thoroughly clarified. This dissertation explores the use of molecular beam epitaxy (MBE) of novel nitride heterostructures grown on bulk substrates. First and foremost, the experimental demonstration of stimulated emission in high aluminum composition heterostructures by optical pumping, a theoretical model to determine optical losses from cavity imperfections, as well as the realization of quasi-vertical diodes exploiting distributed polarization doping as an alternative for hole injection for ultraviolet optoelectronics, are realized, paving the way towards deep-ultraviolet electrically injected laser diodes by MBE. This work follows with the study of semiconductor integration schemes. Dualtronics is introduced, where both faces of a single polar bulk GaN substrate are utilized simultaneously. A monolithic integration of (Al,Ga)N high-electron mobility transistors (HEMTs) on the nitrogen-polar face and blue (In,Ga)N LEDs on the metal-polar face of the same wafer is demonstrated, proving this concept may be applied to effectively combine the unique electronic, photonic, and accoustic functionalities of the nitride semiconductor family. With the recent development of epitaxy on native substrates resulting in semiconductor layers with high crystallinity and low point and extended defect concentrations, in-depth spectroscopic studies are performed. These include the determination of radiative and non-radiative pathways in InGaN based LEDs grown on bulk GaN, as well as the probing of the excitonic fine structure and defect luminescence of nitrogen and aluminum polar AlN grown on bulk AlN substrates. This dissertation concludes with the exploration of the role that transition metal nitrides, most notably AlScN, may play in integration with the existing III-N optoelectronics ecosystem. Fundamental characterization of its structural and (non)linear optical properties and the demonstration of low-loss layers grown on AlN and GaN lead to the development of high-reflectivity lattice-matched AlScN/GaN distributed Bragg reflectors, outperforming the well-studied lattice-matched InAlN/GaN platform. This dissertation suggests that the nitride semiconductor platform has a bright future.