To appear in Discrete and Continuous Dynamical Systems A.-- 56 pages.-- 2000 MSC Classes: 70F25, 70H03, 70H33, 37J60, 53D17.This paper presents a geometric description on Lie algebroids of Lagrangian systems subject to nonholonomic constraints. The Lie algebroid framework provides a natural generalization of classical tangent bundle geometry. We define the notion of nonholonomically constrained system, and characterize regularity conditions that guarantee the dynamics of the system can be obtained as a suitable projection of the unconstrained dynamics. The proposed novel formalism provides new insights into the geometry of nonholonomic systems, and allows us to treat in a unified way a variety of situations, including systems with symmetry, morphisms and reduction, and nonlinearly constrained systems. Various examples illustrate the results.This work has been partially supported by Spanish Ministry of Education and Culture grants MTM2004-7832, BFM2003-01319, MTM2006-03322 and BFM2003-02532. J. Cortés was partially supported by faculty research funds granted by the University of California, Santa Cruz.Peer reviewed