Young, massive stars influence their ambient medium through winds and radiation. The outcome of this feedback depends on the number of massive stars in a star cluster and on the density of the ambient medium. This contribution is based on a comparison of observations to the results of numerical simulations. We discuss the gravitational fragmentation of feedback-driven shells expanding from young stellar clusters. The thin-shell approximation is compared to 3D hydrodynamical simulations with smoothed-particle hydrodynamics and adaptive-mesh refinement codes. We explore the influence of external pressure and propose a thick-shell dispersion relation, where the pressure of the external medium is included. The mass spectrum of the shell fragments is constructed and we speculate about the origin of the deficit of low-mass objects.