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.