2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/76391We show that every non-trivial tame knot or link in R^3 has a quadrisecant, i.e. four collinear points. The quadrisecant must be topologically non-trivial in a precise sense. As an application, we show that a nonsingular, algebraic surface in R^3 which is a knotted torus must have degree at least eight.7 pages. Figures added, retypeset in REVTeX, spelling correctedGeometric Topologytext