This chapter outlines the origin of quantum theory, beginning with Max Planck’s discovery of energy quanta and Einstein’s explanation of the photoelectric effect. It also recounts the historical context that led to the development of Einstein’s Theory of Special Relativity in 1905, which unified space and time into the concept of spacetime pointing out that space and time must be treated at the same footing. A decade later, in 1915, Einstein introduced the General Theory of Relativity, a broader framework that differed significantly from special relativity. Solutions to Einstein’s field equations revealed groundbreaking insights, including the bending of starlight, spacetime singularities, gravitational waves, and the formation of neutron stars and black holes. The theory also predicted the expansion of the universe. Simultaneously, quantum mechanics was rapidly developing. The chapter describes key atomic models that advanced the field of quantum mechanics. It outlines three major formulations of quantum mechanics: Heisenberg’s matrix mechanics (including the uncertainty principle), Schrödinger’s wave mechanics, and Dirac’s relativistic quantum theory. The de Broglie hypothesis of wave-particle duality, regarded as a cornerstone of quantum mechanics, is also discussed. The chapter systematically presents the development of approximate methods for solving quantum systems and concludes with an introduction to relativistic quantum field theory and Dirac’s concept of antiparticles.