In this colloquium I review the scientific potential of a new electron-positron collider with sufficient center-of-mass energy and luminosity to produce large sample of Higgs bosons and top quarks. Prospects for searches complement the hadron collider program in important ways. The key strength of the Higgs/top/EW factory, however, lies in precision measurements of electro-weak processes and Higgs and top quark properties and interactions. The projected precision of these measurements - often a per mille level - improves in an important way on the current state of the art and on the HL-LHC projections. Thus, the Higgs/top/EW factory completes the characterization of the Standard Model of particle physics at the electro-weak scale. I review the projections for key measurements and discuss the strengths and weaknesses of several proposed electron-positron collider projects.