It is well known that, in kinetics, the interaction between dislocations and interstitial solute normally exerts strong solute drag effect on dislocations, leading to strong solution hardening of the metals. However, due to the low mobility of interstitial solute in many metals, thermodynamic aspect of the interaction between dislocations and interstitial solute is often unobservable and omitted. It will be shown in this article by reviewing the H-induced behaviors in metal–H systems, especially the recent progress in Pd–H system that, when the interstitial solute atoms are highly mobile and able to collect in the vicinity of mobile dislocations easily, the scenario will be remarkably different. The interaction between dislocations and these highly mobile interstitial solute atoms, in thermodynamics, will reduce the line energy of dislocations and will facilitate the generation of dislocations, leading to an increase in dislocation density and an enhanced strain hardening of metals upon plastic deformation. However, due to the low mobility of interstitial solute in many metals, thermodynamic aspect of the interaction between dislocations and interstitial solute is often unobservable and omitted. It will be shown under this article by reviewing the H-induced behaviors in metal-H systems, especially the recent progress in Pd-H system that, when the interstitial solute atoms are highly mobile and able to collect in the vicinity of mobile dislocations easily, the scenario will be remarkably different. The interaction between dislocations and these highly mobile interstitial solute atoms, in thermodynamics, will reduce the line energy of dislocations and will facilitate the generation of dislocations, leading to an increase in dislocation density and an enhanced strain hardening of metals upon plastic deformation.