The Jacob’s Ladder is a metaphorical scale that represents the different levels of approximation of the functionals in DFT: as one climbs the rungs, the mathematical complexity and physical accuracy increase, approaching the so-called perfect functional that would exactly describe the behavior of electrons. LDA and GGA are the first steps; at present, work is being done on the Meta-GGA, taking into account the kinetic energy density of the electron. The fourth rung considers an exact exchange. The last step considers both exact exchange and exact correlation.