Emerging versatile two-dimensional MoSi${}_2$N${}_4$ family

The discovery of 2D layered MoSi${}_2$N${}_4$ and WSi${}_2$N${}_4$ without knowing their 3D parents by chemical vapor deposition in 2020 has stimulated extensive studies of 2D MA${}_2$Z${}_4$ system due to its structural complexity and diversity as well as versatile and intriguing properties. Here, a comprehensive overview on the state-of-the-art progress of this 2D MA${}_2$Z${}_4$ family is presented. Starting by describing the unique sandwich structural characteristics of the emerging monolayer MA${}_2$Z${}_4$, their versatile properties including mechanics, piezoelectricity, thermal transport, electronics, optics/optoelectronics, and magnetism is summarized and anatomized. The property tunability via strain engineering, surface functionalization and layered strategy is also elaborated. Theoretical and experimental attempts or advances in applying 2D MA${}_2$Z${}_4$ to transistors, photocatalysts, batteries and gas sensors are then reviewed to show its prospective applications over a vast territory. New opportunities are further discussed and prospects are suggested for this emerging 2D family. The overview is anticipated to guide the further understanding and exploration on 2D MA${}_2$Z${}_4$. (© 2023 Wiley‐VCH GmbH)