Published in: Journal of Physics A: Mathematical and Theoretical, 40 (2007) 12071–12093 (Institute of Physics, ISSN 1751-8121)A geometric approach to time-dependent optimal control problems is proposed. This formulation is based on the Skinner and Rusk formalism for Lagrangian and Hamiltonian systems. The corresponding unified formalism developed for optimal control systems allows us to formulate geometrically the necessary conditions given by Pontryagin's Maximum Principle, providing that the differentiability with respect to controls is assumed and the space of controls is open. Furthermore, our method is also valid for implicit optimal control systems and, in particular, for the so-called descriptor systems (optimal control problems including both differential and algebraic equations).We acknowledge the financial support of Ministerio de Educación y Ciencia, Projects MTM2005-04947, MTM2004-7832, and S-0505/ESP/0158 of the CAM. One of us (MBL) also acknowledges the financial support of the FPU grant AP20040096.Peer reviewed