The atomic clock is the nearest approximation we have to the hypothetical proper clock of the ‘gedanken,’ or thought, experiments that are used to illustrate the new relativistic or gravitational phenomena identified in post-Newtonian studies of gravitation. The recent decades of progress in space technology have made the whole solar system accessible as a laboratory for gravitation, and the atomic clock with stability in the 10−16 domain offers a method of measuring distance and velocity as well as being an experimental device to probe the behavior of time. Experimental gravitation, which is now nearly synonymous to experimental relativity, is reviewed in the context of modern experimental techniques, and the role of the clock is discussed in detail. A catalog of experiments, based on four classes of experiments as described by Misner et al. (1970), is used to identify the particular aspect of gravity that is being tested, and the application of the clock is discussed in each case.