  Hydrogen-induced reactions provide essential information on nuclear
structure, complementary to other experimental probes. For studies at both low
and relativistic incident energy, developments in hydrogen targets have been
performed over the past 10 years in parallel with the development of new
radioactive beams. We present a review of all major hydrogen target
developments related to the study of exotic nuclei with direct reactions in
inverse kinematics. Both polarized and non-polarized systems are presented.
