The elemental group-IVA monolayers (Xenes)Xenes are among the strongest bonded lattices owing to the four valence electronsValence electrons forming two-center two-electron bonds, which fulfill the octet ruleOctet rule. GrapheneGraphene, with the shortest bond and planar honeycomb structureHoneycomb structure, has the largest cohesive energyCohesive energy and highest mechanical stability of the single-atomic layers. For the homologues siliceneSilicene, germaneneGermanene, staneneStanene, and plumbenePlumbene, the larger atom size reduces delocalized π bonding, and an energetically more stable mixed sp2–sp3 hybridizationHybridization generates monolayers with increasing buckling. The contribution of sp3 hybrids to mixed bonding and thus buckling increases from Si to Pb. A decrease in the orbital overlapOrbital overlap leads to weaker covalent bondingCovalent bonding and a drastic decrease in the stiffnessStiffness and ultimate strengthUltimate strength with increasing atom size. While graphene can be prepared as a stable free-standing monolayerFree-standing monolayer, for the other monolayers to date, synthesisSynthesis was only possible on a suitable stabilizing substrate that prevents the normally preferred tetrahedral sp3 bonding of these elements. Group–IVA monolayers play key roles in the active search for a new class of nontrivial topological states. Examples are stanene and plumbenePlumbene with a planar configuration due to strong substrate interaction. Quantum spin Hall (QSH) systems are topological insulators (TIs) that possess edge states of counterpropagating opposite spin states in connection with relativistic spin–orbit coupling and time-reversal symmetryTime-reversal symmetry. Such systems are insulating in the bulk owing to an open band gap; however, they have gapless edges or surface states at the boundary, which are immune to imperfections and geometric perturbations. Since dissipationless transport mechanisms are of extreme interest for the semiconductor industry, this is a field of active research. Spin–orbit coupling (SOC), which strongly increases from grapheneGraphene to plumbene, and time-reversal invariance play essential roles. The realization of large gaps for the heavier homologues of graphene, staneneStanene and plumbenePlumbene with a graphene-like lattice may allow room-temperature applications of topological devices.