In volume one of Einstein's Mass-Energy Equation, we examine the history and philosophical significance of several demonstrations Einstein published for his mass-energy relation, which is often expressed by the iconic equation E = mc2. Our goal is to illustrate how these demonstrations display a clear shift away from a reliance on electromagnetic phenomena culminating in Einstein's 1934 purely dynamic demonstration. Philosophically, this trend signals the importance of recognizing special relativity as what Einstein called a principle theory. Volume two of this work examines the role that Einstein's mass-energy relation played in the development of quantum mechanics and general relativity. We also discuss the first empirical confirmation of E = mc2 and some contemporary debates concerning the philosophical interpretation of this important result.