The single most important concept in all of investing, according to Benjamin Graham and later confirmed by his star student, Warren Buffett, comes down to three simple words: Margin of Safety. What is the margin of safety? How do you calculate it? How important is it to developing a successful value investing strategy? As you’ll see in a moment, the theory behind value investing is that the ultimate return you earn on your investments will be closely related to the size and quality of the margin of safety you build in to your purchasing decisions, whether you are buying shares of Coca-Cola or building a hotel.