Since the Bekenstein-Hawking black hole entropy made its appearance in the seventies, the idea that the ultimate number of degrees of freedom in a region depends on the area of its boundary began to gain currency. The idea sounds surprising. Entropy measures the number of degrees of freedom (“the stuff”). Normally we expect its amount to grow with volume, as with the amount of stuff we can squeeze into a bag or a box.  The research in string theory and loop quantum gravity allowed to derive the “area laws” for black hole entropy  from the first principles, with only a small amount of additional assumptions. At the same time the black hole thermodynamics was developed.