2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/34254General Relativity is contaminated with non-trivial geometries which generate closed timelike curves. These apparently violate causality, producing time-travel paradoxes. We shall briefly discuss these geometries and analyze some of their physical aspects.8 pages, LaTeX2e, Kluwer Academic style files-included. Invited talk at the NATO Advanced Research Workshop, The Nature of Time: Geometry, Physics and Perception, May 21-24, 2002, Tatranska Lomnica, Slovak Republic. Revised versionGeneral Relativity and Quantum Cosmologytext