International audienceHydrogenases are efficient biological catalysts of H2 oxidation and production. Most of them are inhibited by oxygen, and a prerequisite for their use in biotechnological applications under air is to improve their oxygen tolerance. A few bacteria, however, contain hydrogenases that activate H2 even in the presence of O2. Intriguingly, molecular, kinetic and spectroscopic studies lead to assume that different mechanisms might be responsible for the resistance, depending on the enzyme type. In order to better understand the molecular bases of resistance to O2 inhibition, this chapter focuses on the hydrogenases and their reaction with O2 and examines the different strategies to lead to engineer kinetically efficient hydr...