Early attempts at path integrals can be traced back to 1918. A sound mathematical formalism wasn't established until 1921. This eventually lead Richard Feynman to construct a formulation for quantum mechanics, now commonly known as Feynman Integrals. In the core of Path integrals lies the concept of Functional integration. Regular integrals consist of a limiting process where a sum of functions is taken over a space of the function's variables. In functional integration the sum of functionals is taken over a space of functions. For each function the functional returns a value to add up. Path integrals should not be confused with line integrals which are regular integrals with the integration evaluated along a curve in the variable's space. Not very surprisingly functional integrals often diverge, therefore to obtain physically meaningful results a quotient of path integrals is taken.