Abstract : We address the statistical issue of determining the maximal spaces (maxisets) where model selection procedures attain a given rate of convergence. By considering first general dictionaries, then orthonormal bases, we characterize these maxisets in terms of approximation spaces. These results are illustrated by classical choices of wavele model collections. For each of them, the maxisets are described in terms of functional spaces. We take a special case of the issue of calculability and measure the induced loss of performance in terms of maxisets.