The paper reviews general relativity, Einstein's theory of gravitation. Emphasis is put on the invariance groups of general coordinate and local Lorentz transformations. The presentation of the dynamics is based on Schwinger's action principle and the Hamiltonian approach. The machinery of accelerated reference frames in Minkowski space is discussed. Observer-relevant results from post-Newtonian calculations are presented, experimental tests of the foundations and the predictions of the theory are summarized.