Although only a few 2D materials have been predicted to possess ferroelectricity, 2D ferroelectrics are expected to play a dominant role in the upcoming nano era as important functional materials. The ferroelectric properties of 2D ferroelectrics are significantly different than those of traditional bulk ferroelectrics owing to their intrinsic size and surface effects. To date, 2D ferroelectrics have been reported to exhibit diverse properties ranging from bulk photovoltaic and piezoelectric/pyroelectric effects to the spontaneous valley and spin polarization. These properties are either dependent on ferroelectric polarization or coupled with it for easy electric control, thus making 2D ferroelectrics applicable to multifunctional nanodevices. At present, cumulative efforts are being made to explore 2D ferroelectrics in theories, experiments, and applications. Herein, such theories and methods are briefly introduced. Subsequently, intrinsic and extrinsic origins of 2D ferroelectricity are separately summarized. In addition, invented or laboratory‐validated 2D ferroelectric‐based applications are listed. Finally, the existing challenges and prospects of 2D ferroelectrics are discussed. 2D ferroelectrics are expected to play a dominant role in the upcoming nano era as important functional materials. Relevant theories, as well as intrinsic and extrinsic 2D ferroelectrics, are outlined. Further, the existing challenges and prospects of the exploration in 2D ferroelectrics are also discussed, which should provide unprecedented opportunities in the fields of electronics, spintronics, optoelectronics, and valleytronics.