  Tests of gravity performed in the solar system show a good agreement with
general relativity. The latter is however challenged by observations at larger,
galactic and cosmic, scales which are presently cured by introducing "dark
matter" or "dark energy". A few measurements in the solar system, particularly
the so-called "Pioneer anomaly", might also be pointing at a modification of
gravity law at ranges of the order of the size of the solar system. The present
lecture notes discuss the current status of tests of general relativity in the
solar system. They describe metric extensions of general relativity which have
the capability to preserve compatibility with existing gravity tests while
opening free space for new phenomena. They present arguments for new mission
designs and new space technologies as well as for having a new look on data of
existing or future experiments.
