Hydrogen-induced plasticity degradation is a major feature of hydrogen embrittlement. However, we still know very little about how hydrogen affects on the behavior of dislocations, the predominant carrier of plasticity in metals, primarily because of the lack of effective experimental methods and results at the level of individual dislocations. In the classic work of Birnbaum, Robertson, and coworkers, the experimental setup was to change the hydrogen atmosphere during or after dislocation slip in an environmental transmission electron microscope (ETEM). However, there are some unclear points regarding the experimental setup they used to determine the hydrogen effect on dislocation mobility. Prior to the introduction of hydrogen, the observed dislocations were generally in the relaxed state, making it impossible to observe hydrogen-induced drag or pinning effects. Second, if thousands of pascals of H2 gas were flooded into or pumped out of the ETEM chamber within a few seconds, this could have altered either the local distribution of internal stresses or the external loading, which had been assumed to be constant. These effects could have induced dislocation motion due to their high sensitivity to stresses. Since dislocations eventually move by pulling pinning points on the film surfaces, the speed of a dislocation would be largely determined by the pinning effect at both dislocation ends. However, dislocation pinning can be modified by possible hydrogen-induced reduction of surface oxides or removal of organic impurities from the surface under electron beam illumination. These considerations motivate us to perform well-controlled experiments capable of significantly reducing the effects of these factors, especially with respect to uncontrolled surface pinning of dislocations. Here, I will present a new experimental setup based on the in situ ETEM technique and quantitative nanomechanical testing that allows a well-controlled evaluation of the hydrogen effect on dislocation behavior in metals. The corresponding results of experiments with aluminum and iron will be presented and open to discussion in this talk. (Xie, 2016 Nat. Comm., Xie 2023 Nature Materials).