Einstein's special theory of relativity was published in 1905. It stands as one of the greatest intellectual achievements in the history of human thought. Einstein described the equivalence of mass and energy as "the most important upshot of the special theory of relativity" (Einstein, 1919). In this paper, we will discuss the evolution of the concepts of Einstein's mass-energy relationship during these 100 years. Challenges, issues and implications for curriculum development, instruction and assessment are also discussed.