This important paper is destined to be cited more often for the innovative techniques that are introduced rather than for the specific results that are proved. The proofs of the asymptotic formulas and the geometric explanation of the canard phenomenon are all highly nontrivial, while making use of center manifold theory, normal forms, foliations, Abelian integrals, asymptotic series and desingularization by weighted blow-up. The desingularization technique is particularly interesting and involves a construction for blow-up of parametrized families of vector fields, a rather far-reaching generalization of the blow-up construction for a fixed singularity. In fact, the blow-up of a parametrized family of vector fields yields a new geometric object that the authors call a foliated local vector field. This concept generalizes the idea of rescaling a differential equation and thus should be very useful in the analysis of many other singular perturbation problems.