‘Relativity theory’ or simply ‘Relativity’ is the standard name of two quite different, yet subtly related theories put forward by Albert Einstein in 1905 and 1915, respectively. The first, Special Relativity (SR), rescued the Maxwell equations from seemingly catastrophic experimental results by making deep changes in the basic concepts and laws of Newtonian mechanics. The second, General Relativity (GR), solved the problem of reconciling SR with Newton's theory of gravity by transcending them both. For those of us who cherish physicomathematical theories more for their inherent beauty than for their transient accuracy, SR and GR remain unmatched. Moreover, to this day, they have enjoyed tremendous empirical success. SR is corroborated daily in every high-energy lab. GR accounts for all the phenomena Newton classified as gravitational just as well or even better than his theory. Moreover, it provides an amazing gravitational explanation of other phenomena – such as the systematic shift in the spectrum of light from distant galaxies and the pervasive microwave background radiation –, which nobody even suspected c. 1910 and which would not easily fit in a Newtonian framework.

This is not the place to deal even superficially with the many fruitful applications of SR and GR. Our attention must go to their con-ceptual problems and their philosophical significance. The latter has been judged differently by different authors. In my view, it lies chiefly in the fact that both theories are exemplary cases of far-reaching conceptual change in fundamental physics, firmly rooted in the tradition they go beyond.