This chapter discusses the fundamental theories and understanding of the field emission phenomenon from both classical and quantum perspectives. For this, the basic idea of free electron theory by Drude and Lorentz has been discussed here, as well as its advantages and drawbacks. The concept of quantum statistics and the density of states are introduced and further extended to deduce the Richardson equation of thermionic emission and the Schottky effect. The basic current–voltage relationship observed by Millikan and Lauritsen (M-L equation) is presented, and it has been extended to derive the most widely used Fowler–Nordheim equation (F-N equation). Obviously, there are many modifications in the original F-N equation that have also been included in detail. This chapter also comments on the maximum field range over which the F-N equation is valid and mentions some of the peculiarities that are often observed when one uses this equation. In this regard, the effect of Fermi surface structure and tunneling of many particles are extensively discussed. Finally, a brief summary of all the above-mentioned discussions is provided.