Most of the information presently available on binary group IVA–VA monolayers with the stoichiometry AB (A = C, Si, Ge, Sn, and Pb; B = N, P, As, Sb, and Bi) is based on first-principles calculations. Monolayers of these compounds with the chemical formula A2B2 exist as hexagonal four-layer assemblies with the configurations B–A–A–B, named the α and β phases, with the space groups P-6m2 and P-3m1, respectively. Cutting the covalent A–A bonds yields two buckled monolayers. Their unit cell consists of two A and two B atoms. Furthermore, a less symmetric monoclinic phase consisting of hexagons and pentagons has been synthesized. These and other allotropes often possess similar binding energies due to the similarity of the bonding arrangements, and therefore the most stable phase often is not known. Notably, enormous differences in the calculated cohesive or binding energies exist in the literature. Some structures possess a high anisotropy owing to differences in chemical bonding in the armchair and the zigzag directions. In particular, nonlinear mechanical properties such as the ultimate strength and strain depend sensitively on anisotropic bonding. The monolayers or few-layer sheets are direct or indirect semiconductors with band gaps of 0.3–5.0 eV; in a few cases, they have metallic character. Usually, these 2D materials are excellent absorbers of visible and ultraviolet light. With their strain-tunable bandgaps and band edges, they are of interest for optoelectronic applications and as photocatalysts, e.g., for photocatalytic decomposition of water. The controlled mechanical or liquid-phase exfoliationLiquid-phase exfoliation of high-quality monolayers and few-layer assemblies from synthesized layered bulk crystals is difficult and has been successful in only a few cases. The synthesisSynthesis of these compounds by bottom-up methods such as chemical vapor deposition (CVD)Chemical Vapor Deposition (CVD) andMolecular Beam Epitaxy (MBE) molecular beam epitaxyEpitaxy (MBE) is still lacking.