This thesis focuses on the thermal transport properties of three ultrawide bandgap (UWBG) semiconductors: wurtzite aluminum nitride (w-AlN), hexagonal boron nitride (h-BN), and aluminum scandium nitride (AlScN). The continued scaling down of device dimensions and operation in high-voltage or high-frequency regimes aggravates thermal loads, and effective heat dissipation has become one of the most critical challenges in device performance and reliability. w-AlN has attracted increasing attention for high-power and high-temperature operations due to its high piezoelectricity, UWBG, and large thermal conductivity k. The k of epitaxially grown AlN on foreign substrates has been investigated, however, no thermal studies have been conducted on homoepitaxially grown AlN. In this thesis, the thickness dependent k and thermal boundary conductance G of homoepitaxial AlN thin films were systematically studied using the optical pump-probe method of frequency-domain thermoreflectance (FDTR). Our results show that k increases with thickness and k values are among the highest reported for film thicknesses of 200 nm, 500 nm and 1 μm, with values of 71.95, 152.04, and 195.71 W/(mK), respectively. Our first-principles calculations show good agreement with our measured data. Remarkably, the G between the epilayer and the substrate show some of the record-high values of 328, 477, 1180, and 2590 MW/(m^2 K) for sample thicknesses of 200 nm, 500 nm, 1 μm, and 3 μm, respectively. The high k and ultra-high G of homoepitaxially gown AlN are very promising for efficient heat dissipation, which can inform device design and advance applications in micro-electromechanical systems (MEMS), ultraviolet photonics, and high-power electronics. In terms of h-BN, its unique features including its atomically smooth surface, large critical electric field, and large electronic band gap make it ideal for thin film microelectronics and as an UWBG semiconductor. Owing to weak van der Waals interactions between layers, h-BN has a highly anisotropic thermal conductivity. The in-plane thermal conductivity of h-BN has been extensively studied, yet the only measured data of cross-plane thermal conductivity (k_⊥) is for exfoliated h-BN films. Exfoliation from bulk crystals is not a sustainable method for mass production of h-BN due to its low repeatability, low yield, poor control of sample thickness and limitation to small areas. Thus, there is a need to investigate the thickness dependent k_⊥ for a practical growth method, which provides reliable and large-area h-BN with control of film thickness, such as using pulsed laser deposition (PLD). We grew h-BN using PLD at 750 ℃ and observed a monotonic trend of k_⊥ as thickness increases from 30 nm to 300 nm, varying from ~1.5 to ~0.2 W/(mK). Remarkably, we observed record-high k_⊥ value for h-BN at 30 nm thick. Our results offer the first insights into the k_⊥ of PLD-grown films for electronics applications.Lastly, aluminum scandium nitride (AlScN) has been receiving increasing interest for radio frequency microelectromechanical systems because of their higher achievable bandwidths owing to the larger piezoelectric response of AlScN compared to AlN. However, alloying scandium (Sc) with aluminum nitride (AlN) significantly lowers the thermal conductivity of AlScN due to phonon alloy scattering. Self-heating in AlScN devices potentially limits power handling, constrains the maximum transmission rate, and ultimately leads to thermal failure. We grew plasma assisted molecular beam epitaxy (PAMBE) AlScN on AlN-Al2O3 substrates and MEBE AlScN on GaN-Al2O3 substrates; both indicated a comparable thermal conductivity to current work on AlScN grown on Si substrates, but with significantly thinner films. These findings help enhance the understanding of the thermal transport properties of the AlScN and its corresponding nitride heterostructure and ultimately have significant implications for the use of AlScN in acoustic, power electronic, and high-speed microwave applications where heat management is critical to device performance.