Results of microscopic modeling of semiconductor vertical-cavity surface-emitting lasers (VCSELs) are discussed. The treatment of the laser as a nonequilibrium many-body system provides a detailed understanding of the various processes that determine the laser output and the electron-hole-plasma excitation. It is shown that the transient gain dynamics are strongly influenced by nonequilibrium carrier effects. These gain dynamics together with the cavity design determine the delayed onset and the temporal and spectral shape of the laser output. The theory is evaluated to investigate how the laser output properties can be controlled in terms of (1) excitation conditions of the VCSEL, (2) the mirror design, which allows us to change the cavity quality and the resonance frequency, and (3) the number and position of semiconductor quantum wells as active material.