In 1905, Einstein published his paper on the special theory of relativity. Although Lorentz and Poincaré had introduced a number of the essential feature of the special theory, Einstein's' paper went much deeper and showed that we live in four-dimensional space-time. A consequence of this approach was the derivation of the mass-energy relation E = mc2. The route to the special theory of relativity was not straightforward, but Einstein's approach was quickly adopted and the mass energy relation confirmed by the Cockcroft and Walton experiment of 1932. In this chapter, the theory of special relativity is developed through the use of four-vectors, providing a robust framework for carrying out calculations in special relativity.