  Lepton-hadron colliders that use a proton or nucleus beam of current and
future hadron colliders and let it collide with an electron beam from a newly
built electron accelerator bring attractive physics programs which are strong
and complementary to the hadron collider physics. Machine development for
Energy Recovery LINAC and physics performance studies of such electron-hadron
colliders, specifically the LHeC that uses the existing LHC beam and FCC-eh
that is an option of Future Circular Collider program, are ongoing and reviewed
in this article.
