This book is divided into two parts. In the first part we introduce the foundations of special relativity, such as, the inertial frame of reference, the definition of simulataneity, and Einstein's two basic hypotheses. We give the main relativistic effects, e.g. the relativity of simultaneity, velocity addition, length-contraction, the apparent shape of a moving body, time-dilation, Doppler effect, and the Thomas precession, In particular, the simultaneity problem and slow transport of clocks are investigated in detail by means of the test theories of special relativity. In the second part, variant types of experiments performed up to now are analyzed and compared to the predictions of special relativity. This shows that the experiments are a test of the two-way speed of light, but not of the one-way speed of light.