"Constrained systems: a unified geometric approach" A general geometric framework is devised in order to contain the pre- symplectic and lagrangian formalisms as particular cases. We call these objects *constrained dynamical systems* since their dynamics usually lead to *constraints*. Their most elementary properties are studied, and several related structures, especially morphisms, are defined. In particular, a stabilization algorithm is performed. As a byproduct, the dynamics and constraints of the lagrangian formalism (with the "second order condition") are intrinsically obtained.