The group of IIIA–VA monolayers of MX (M = B, Al, Ga, In, and Tl; X = N, P, As, Sb, and Bi) with 25 compounds contains several well-studied monolayers, and some of them have already been found to be applicable in electronic and optoelectronic devices. The main available information comes from first-principles calculations, which cover the structural data, cohesive binding energies, electronic properties, and mechanical behavior of these compounds. The electronic structure of group–IIIA atoms is (ns2np1) with three valence electrons, whereas that of group–VA atoms is (ns2np3) with five valence electronsValence electrons. These configurations leave an unoccupied pz orbital in the group–IIIA atom and a lone pair of electrons in the group–VA atom, forming π bonds. The preferred sp3 bonding in the bulk and the large interlayer interaction energy in few-layer assemblies make the synthesisSynthesis of monolayers demanding. The different electronegativitiesElectronegativities cause mixed covalent-ionic bondingCovalent-ionic bonding of the heteroatomic compounds. Owing to their greater electronegativity, the valence electron distribution of the polar bonds is concentrated on the group-VA atoms. Importantly, planar group-IIIA nitrides exhibit large ionic contributions to bonding, inducing wide bandgaps, which are of interest in lighting and photovoltaicsPhotovoltaics. Owing to the small size of the boron and nitrogen atoms, strong overlap of π orbitals occurs that stabilizes planar sp2 hybrids, e.g., in h-BN. The other boron‒pnictogen monolayers show only minor deviations from planarity for the heaviest compound h-BBi, while the other compounds have a stable planar configuration. Based on their electronic properties, these monolayers provide fundamental building blocksBuilding blocks for 2D heterostructures and are promising materials for nanoelectronic and optoelectronic devices. Most of the monolayers possess thermodynamic, thermal, and dynamic stabilityDynamic stability. The stiffnessStiffness and ultimate strengthUltimate strength decrease enormously from h-BN to the heaviest compound, h-TlBi.