Abstract: Recently, researchers from the Department of Physics at Tsinghua University and collaborators have made important progress in the study of Weyl semimetals. They have reported the first experimental observation of topological Fermi arcs in a type-II semimetal MoTe₂, thus confirming the violation of Lorentz invariance in type-II Weyl fermions in a real material. The results were published online in Nature Physics on Sept. 6, 2016. This is a major progress in the field of condensed matter physics, after the realization of type-I Weyl fermions. What are type-II Weyl fermions? And how is it different from type-I Weyl fermions? Here we shall give a brief introduction.