Molybdenum and tungsten enzymes are almost omnipresent in living organisms; they catalyze a large variety of reactions on a large range of substrates, and are involved in a number of key cellular functions, like bacterial respiration, carbon, sulfur, and nitrogen cycles, and detoxification. Most of these enzymes catalyze redox reactions, and they have successfully been connected to electrodes, in order to not only build biotechnological devices like biosensors or biofuel cells but also to learn about their catalytic properties, using protein film voltammetry, in which the electron transfer is direct and the enzymatic activity can be monitored as an electrical current. This chapter is an exhaustive review of the uses of direct electrochemistry for the study of molybdenum and tungsten enzymes.