This book highlights the numerous important contributions that Einstein made to physics--aside from his relativity theories--and places each of his achievements in the corresponding context, referring en route to the original sources. There are very few publications devoted to Einstein's work outside of relativity. This book aims to fill the gap by exploring the scope of Einstein's contributions on topics including molecular forces, thermostatistics, the photoelectric effect, Brownian motion, molecular currents, critical opalescence, energy quanta, dual structure of radiation, introduction of the photon, and the formulation of the first quantum statistics. The book pays special attention to Einstein's scepticism toward certain ideas that came to light alongside Schrödinger's first formulation of wave mechanics in 1926, also addressing his doubts regarding the probabilistic interpretation of the quantum formalism, an issue closely connected with the hidden variable theories and their implications. The author discusses the early hidden variable theories, whose appearance was largely a result of Einstein's criticism of the orthodox interpretation of quantum formalism. Finally, in an appendix, the author explores the controversy about the possible contribution that Mileva Marić, Albert Einstein's first wife, may have made to some of her husband's main scientific achievements.