Motivated by protein sequencing, we consider the problem of reconstructing a string from the compositions of its substrings. We provide several results, including the following. General classes of strings that cannot be distinguished from their substring compositions. An almost complete characterization of the lengths for which reconstruction is possible. Bounds on the number of strings with the same substring compositions in terms of the number of divisors of the string length plus one. A relation to the turnpike problem and a bivariate polynomial formulation of string reconstruction.