Hexagonal boron nitride (h-BN), a member of 2D materials, has an analogous crystal structure to graphene, yet, is an insulator having an indirect bandgap of ≈6 eV. [16] h-BN has been explored as the "ideal" substrate and excellent encapsulant [17] for other 2D materials including graphene and transition metal dichalcogenides (TMDs) due to its excellent insulating property, atomically flat surface free of dangling bonds, [18] and charged impurities, and high thermal conductivity. [19,20] Encapsulating other 2D materials with h-BN effectively protects them from local electrical and chemical environment, enabling them to manifest their theoretically expected "intrinsic" properties. In addition, h-BN has been utilized as passive components in 2D vdW electronic devices, such as gate dielectrics and tunneling barriers.Recently, h-BN is attracting intensive research interests, motivated by the variety of outstanding properties, it shows across the fields of quantum optics, [21] electronics, and optoelectronics. [22] For example, despite its indirect-bandgap nature, h-BN showed deep-ultraviolet (DUV) emission with exceptionally high internal quantum efficiency (IQE) comparable to or even higher than single-crystalline direct-bandgap semiconductors. Defects in h-BN are efficient [21,23] and stable sources for single-photon emission over a wide range of wavelengths, from near-infrared (NIR) [24] to nearultraviolet (NUV), [25] at room temperature. The giant lightmatter interaction and strong band-edge absorption offered by h-BN enable the realization of solar-blind DUV photo detection by using h-BN as the active layer where DUV photons are effectively absorbed and create electrical carriers. h-BN has also been adopted as active media for low energy electronic devices, including nonvolatile resistive switching (RS) memory and radio-frequency (RF) devices, in addition to passive components for field-effect transistors (FETs), effective diffusion barriers, and low-k interlayer dielectrics (ILDs).This article aims to review the most recent discoveries of extraordinary properties of h-BN and advancements in emerging photonic and electronic applications. Although there are several well-written review articles on h-BN, [26][27][28][29] this review article focuses on the most recent theoretical discoveries, such as the stacking sequences of h-BN and related optical properties, and the elucidation on how an indirect-bandgap h-BN exhibits strong emission as efficient as a direct-bandgap Hexagonal boron nitride (h-BN), an insulating 2D layered material, has recently attracted tremendous interest motivated by the extraordinary properties it shows across the fields of optoelectronics, quantum optics, and electronics, being exotic material platforms for various applications. At an early stage of h-BN research, it is explored as an ideal substrate and insulating layers for other 2D materials due to its atomically flat surface that is free of dangling bonds and charged impurities, and its high thermal conductivity. Recent discoveries of st...