Inspired by the great development of graphene, more and more research has been conducted to seek new two-dimensional(2D) materials with Dirac cones. Although 2D Dirac materials possess many novel properties and physics, they are rare compared with the numerous 2D materials. To provide explanation for the rarity of 2D Dirac materials as well as clues in searching for new Dirac systems, here we review the recent theoretical aspects of various 2D Dirac materials, including graphene, silicene, germanene,graphynes, several boron and carbon sheets, transition-metal oxides(VO2)n/(TiO2)m and(CrO2)n/(TiO2)m, organic and organometallic crystals, so-MoS2, and artiicial latices(electron gases and ultracold atoms). heir structural and electronic properties are summarized. We also investigate how Dirac points emerge, move, and merge in these systems. he von Neumann–Wigner theorem is used to explain the scarcity of Dirac cones in 2D systems, which leads to rigorous requirements on the symmetry,parameters, Fermi level, and band overlap of materials to achieve Dirac cones. Connections between existence of Dirac cones and the structural features are also discussed. Inspired by the great development of graphene, more and more research has been conducted to seek new two-dimensional (2D) materials with Dirac cones. Although 2D Dirac materials have many novel properties and physics, they are rare compared with the numerous 2D materials. To provide explanation for the rarity of 2D Dirac materials as well as clues in searching for new Dirac systems, here we review the recent theoretical aspects of various 2D Dirac materials, including graphene, silicene, germanene, graphynes, several boron and carbon sheets, transition -metal oxides (VO2) n / (TiO2) m and (CrO2) n / (TiO2) m, organic and organometallic crystals, so-MoS2, and artiicial latices (electron gases and ultracold atoms) . We also investigate how Dirac points emerge, move, and merge in these systems. He von Neumann-Wigner theorem is used to explain the scarcity of Dirac cones in 2D systems, which leads to rigorous requirements on the symmetry, para meters, Fermi level, and band overlap of materials to achieve Dirac cones. Connections between existence of Dirac cones and the structural features are also discussed.