This book is about Maxwell's electromagnetic theory of light. First, it is a fully relativistic theory without having a non-relativistic limit. There arise many difficulties in quantising the electromagnetic field and in the physical interpretation of the wavefunction of its quanta. Further, the first quantisation of the Maxwell equations similar to quantisation of classical mechanics by the Schrodinger method, has not been discussed in most books on quantum mechanics. Second, the Maxwell field is the simplest gauge field possessing symmetry with respect to Poincare group of transformations in addition to scale, duality and special conformal transformations whose local versions give rise to new interaction of photons through new gauge fields. One of these gauge fields couples to the spin density of the photon and other particles and can bind fermion-antifermion pairs to give transverse photons. Another interesting aspect of the electromagnetic field is its coherence properties and their interpretation in terms of quantised theory.