
“Compound interest is the most powerful force in the universe.”
Variants: "... is man’s greatest invention" and "... is the eighth wonder of the world".
May add: "He who understands it, earns it; he who doesn’t, pays it."
This Snopes article http://www.snopes.com/quotes/einstein/interest.asp concluded that its status was uncertain, while this post from The Quote Investigator http://quoteinvestigator.com/2011/10/31/compound-interest/ concludes it is most likely a false attribution, since variants of the quote date back to at least 1916, with the early variants not being attributed to Einstein.
Disputed