<p>Two-dimensional materials, such as graphene, hexagonal boron nitride and transition metal dichalcogenides, are normally limited by the known 3D bulk materials. The design and synthesis of entirely new 2D materials, particularly van der Waals (vdW) layered materials, would significantly expand the properties and functionalities of 2D materials. In 2020, a novel vdW layered material, MoSi₂N₄, was synthesized by passivating the surface of 2D non-layered molybdenum nitride with the addition&#xa0;of&#xa0;elemental silicon, which has since opened up a new vdW materials family with the general formula MA₂Z₄. To date, over a hundred MA₂Z₄ materials and their derivatives have been predicted, in addition to the synthesized MSi₂N₄ (M = Mo, W), encompassing metals, semiconductors, superconductors, topological insulators, ferroelectrics and ferromagnets, owing to the diversity of elements and structures in MA₂Z₄. Such materials exhibit a variety of exceptional electronic, optical, thermal, mechanical, ferroelectric and magnetic properties, and they are promising for applications in electronic and optoelectronic devices, electrocatalysis, photocatalysis and batteries. Over the past 4 years, the MoSi₂N₄ materials family has rapidly emerged as a key research frontier in materials science. In this Review, we summarize recent advances in the investigation of materials in the MoSi₂N₄ family, covering their crystal structure, synthesis methods, fundamental properties and potential applications, and provide an outlook on future research directions.</p>