After explaining the mechanism producing double diffusion, its representation analytically is developed and applied to linear stability analysis to determine conditions for double diffusion to occur. Laboratory observations of salt fingers are summarized, as well as the existence of thermohaline staircases produced by fingers in the thermocline. A similar development for the diffusive regime includes layering in Lake Kivu and in polar regions. Lastly, the role of double diffusion in thermohaline intrusions is explored.