GAMERA handles spectral modeling of non-thermally emitting astrophysical sources in a simple and modular way. It allows the user to devise time-dependent models of leptonic and hadronic particle populations in a general astrophysical context (including SNRs, PWNs and AGNs) and to compute their subsequent photon emission. GAMERA can calculate the spectral evolution of a particle population in the presence of time-dependent or constant injection, energy losses and particle escape; it also calculates the radiation spectrum from a parent particle population.