Researchers have proven sharp upper and lower bounds for the approximation of Sobolev functions by sums of multivariate ridge functions.The order of approximation asymptotically behaves as n^(-r/(d-l)), where r is the regularity of the Sobolev functions to be approximated.The lower bound even holds when approximating L^∞-Sobolev functions of regularity r with error measured in L^1.The upper bound applies to the approximation of L^p-Sobolev functions in L^p for any 1 ≤ p ≤ ∞.