Einstein’s publication of the theory of special relativity (SR) in 1905 (Einstein, Ann Phys 17(10):891–921, 1905) came at a time when physics was undergoing dramatic changes through revolutionary developments within only a few decades. Besides special relativity, these include the theory of general relativity, and the final formulation of quantum mechanics. The theory of special relativity revolutionized the concept of space and time. Before, space and time had been treated as strictly separate entities; now special relativity combined them in a four-dimensional spacetime. In this chapter we shall highlight—not necessarily in historically correct order—a few essential experiments whose results were in blatant contradiction to the current theory at the time. These results motivated the formulation of SR, which after all could resolve the seeming contradictions.