Physics potential and development of μ+μ− colliders

We review the physics motivation for two different energy μ+μ− colliders: 200×200 GeV and 2×2 TeV. We also address the major components of such a collider and the uncertainty in the luminosity. Study of μ± cooling in materials as well as crystals are discussed. Some discussion is given concerning the μ±/π± hadron source.