2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/49148We discuss the existing limits on top flavour-changing neutral couplings in models with new vector-like quarks. Large hadron and e+ e- colliders can improve these bounds by more than one order of magnitude.LaTeX 2e, 5 pages, uses JHEP.cls. Talk given by F. A. at the Corfu Summer Institute on Elementary Particle Physics, 1998High Energy Physics - Phenomenologytext