  The discovery of two-dimensional (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$_4$, we summarize
and anatomize their versatile properties including mechanics, piezoelectricity,
thermal transport, electronics, optics/optoelectronics, and magnetism. 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. We further discuss new opportunities and suggest prospects for
this emerging 2D family. The overview is anticipated to guide the further
understanding and exploration on 2D MA$_2$Z$_4$.
