ABSTRACT. If A is a Lie algebroid over a foliated manifold (M, F ), a foliation of A is a Lie subalgebroid B with anchor image T F and such that A/B is locally equivalent with Lie algebroids over the slice manifolds of F . We give several examples and, for foliated Lie algebroids, we discuss the following subjects: the dual Poisson structure and Vaintrob's supervector field, cohomology and deformations of the foliation, integration to a Lie groupoid. In the last section, we define a corresponding notion of a foliation of a Courant algebroid A as a bracket-closed, isotropic subbundle B with anchor image T F and such that B ⊥ /B is locally equivalent with Courant algebroids over the slice manifolds of F . Examples that motivate the definition are given.