The constancy of the laws of physics is an essential ingredient in the development of science and is implicitly assumed in the application of physics to the interpretation of astronomy and geology. This article reviews the present status of the tests of the temporal invariance of the basic interactions of physics. It reaches the conclusion that experimental tests are consistent with no time variation of the gravitational constant, but points to a fundamental difficulty in interpreting such results. The most precise tests of the temporal variation of the electromagnetic interaction together with the strong and weak nuclear forces are supplied by an analysis of neutron capture cross-sections in the spontaneous fossil nuclear reactor established in Gabon two thousand million years ago.