Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for S XI. The configurations included are 2s22p2, 2s2p3, 2p4, 2s22p3l, and 2s22p4l (l=s, p, d), giving rise to 72 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies, 32, 60, 90, 120, and 150 ryd. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates, statistical equilibrium equations for level populations are solved. The effects of resonances, proton rates, photoexcitation, and cascades on level populations have been investigated. The predicted S XI line intensities are compared with EUV and UV observations of the quiet and active Sun.