Ten years ago, the ATLAS and CMS experiments at the CERN Large Hadron Collider discovered the Higgs boson. In the intervening time, these experiments have discovered many Higgs boson processes and have given strong evidence that this particle is the origin of the masses of quarks, leptons, and vector bosons, as expected in the particle physics Standard Model. Still, the mysteries of the Higgs boson are far from solved, and our lack of understanding of the Higgs affects all other open issues in particle physics. An obvious way to learn more is to measure the couplings of the Higgs boson with high precision. The level of precision required calls for a dedicated collider that can study the Higgs boson in e+e- annihilation. I will describe the capabilities and physics opportunities of such a machine. The proposals to realize this opportunity include both circular and linear colliders. I will briefly review the five proposals now on the table.
