The hydrogen molecule is considered the simplest stable molecule of hydrogen. It is a widely used chemical in organic synthesis processes, acting as a reductive reagent. However, the high bond enthalpy of H-H and a dipole moment of zero results in the high stability of dihydrogen. Both direct homolytic fission and heterolytic fission of the H-H bond are difficult. So, there is a need to find effective methods to activate hydrogen molecules. Considering the electron configurations (a partially filled valance shell) of transition metals, there is often a great potential for a transition metal to be an efficient catalyst. The first case of H2 splitting dates back to the end of the 19 th century. Transition metal and alloys, as well as metal complexes, are widely used in the field of activation of hydrogen molecules. Transition metal complexes have offered a relatively well-developed method for the activation of hydrogen molecules. However, there is a promising alternative based on divalent main-group compounds. This method also provides the potential to realize the metal-free catalysis during the activation process of hydrogen molecules, which is able to overcome some limitations of metallic catalysts (e.g., low selectivity, the difficulty of recycling catalysts, and negative effect on the environment). Another metal-free catalysis approach is using frustrated Lewis pairs (FLPs), a new methodology to the activation of hydrogen first reported by Welch et al in 2006. In this review, we will discuss the mechanism of these four methods and make comments on their advantages and limitations, trying to provide a general understanding of hydrogen molecules activation and its future development.