 To illustrate these changes, Section 1.1 reviews the concepts of mass, momentum and kinetic energy in Newtonian physics. In Section 1.2, we begin exploring the concepts of mass and energy in special relativity and state the notation we use in all further discussions, a step we must make explicit as discussions of the equivalence of mass and energy have historically used different notations, which can lead to conceptual confusion. In Section 1.3, we discuss the physical significance of mass-energy equivalence as it applies to the analysis of bodies (construed as indivisible wholes) and idealized versions of composite systems, while in Section 1.4 we discuss mass and energy in atomic physics. In Section 1.5, we present an elementary derivation of Einstein’s famous result with the hope that our readers will get a sense of “why” mass and energy are equivalent according to special relativity. Finally, in Section 1.6, we consider the relationship between the equivalence of mass and energy codified in the equation \(E_o = mc^2\) and the nearly identical, though conceptually quite different, ubiquitous equation \(E = mc^2\). 