2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/34126We review some old and new results about strict and non strict hyperbolic formulations of the Einstein equations.To appear in the proceedings of the first Aegean summer school in General Relativity, S. Cotsakis ed. Springer Lecture Notes in PhysicsGeneral Relativity and Quantum Cosmologytext