Hydrogen sulfide (HS) was once considered to have only toxic properties, until it was discovered to be an endogenous signaling molecule. The effects of HS are dose dependent, with lower concentrations being beneficial and higher concentrations, cytotoxic. This scenario is especially true for the effects of HS on mitochondrial function, where higher concentrations of the gasotransmitter inhibit the electron transport chain, and lower concentrations stimulate bioenergetics in multiple ways. Here we review the role of HS in mitochondrial function and its effects on cellular physiology.