The elements of group IVA–VIA compounds or group–IVA monochalcogenides with four valence electronsValence electrons (ns2px2) for the IVA elements and six valence electronsValence electrons (ns2px2py2) for the VIA elements are direct neighbors of group–VA elements. Therefore, these binary compounds are their isoelectronic counterparts. They form the monolayers MX (M = C, Si, Ge, Sn, and Pb; X = O, S, Se, Te, and Po). The most stable structures of these monolayers are the puckered and buckled configurations, where the group–IVA atoms form three bonds with group–VIA atoms and vice versa, and both the group–IVA atoms and the chalcogen atoms form sp3-like hybrids and the chalcogen atoms possess a lone electron pairLone electron pair. Owing to the different electronegativitiesElectronegativities and atom sizes of the constituents, the binary monolayer structures lack centrosymmetryCentrosymmetry. The stereo-chemically active lone electron pairs are the source of structural distortions leading to the anisotropy of thermal expansion and the distinctive elastic, piezoelectric, optical, and optoelectronic properties of this group of compounds. Of enormous practical interest are the distorted orthorhombic crystals consisting of an asymmetric washboard-like (aw) structure with the space group Pmn21. Up to now, the large exfoliation energyExfoliation energy has hindered the realization of single layers by versatile mechanical exfoliationMechanical exfoliation methods. Only bottom-up techniques, such asMolecular Beam Epitaxy (MBE) molecular beam epitaxyEpitaxy (MBE) and chemical vapor deposition (CVD)Chemical Vapor Deposition (CVD), allowed the successful synthesisSynthesis of selected monolayers such as SnSe, SnTe, and PbTe. The aw phases of the four compounds GeS, GeSe, SnS, and SnSe have attracted enormous attention owing to their high mechanical flexibility, high carrier mobilityCarrier mobility, low lattice thermal conductivity, and strong absorbance of visible light. These properties qualify the 2D materials for potential applications in optoelectronicsOptoelectronics, thermoelectrics, piezoelectric devices, photodetectorsPhotodetector, and gas sensorsGas sensor. Remarkable phenomena related to in-plane strain include giant piezoelectricityPiezoelectricity and multiferroicityMultiferroicity, combining ferroelastic and ferroelectric behavior.