Observations of Gamma-Ray Bursts (GRBs) with the Fermi Gamma Ray Space Telescope open a new window in the understanding of their prompt emission. With data sets from instruments prior to Fermi, GRB prompt emission spectra in the keV-MeV energy range were adequately fit with the empirical Band function, which consists of two power laws (PLs) smoothly connected at a break energy.

While various works suggested deviations from this model, spectral analysis over the broad energy range of Fermi breaks this paradigm and reveals that GRB spectra are sometimes best fit with a combination of multiple components such as a Band function and a PL, a Band function and a thermal component or a Band function, a PL and a thermal component.

While the parameters of the empirical Band function often challenge the physical models in the Band-only scenario, they can be more easily reconciled with synchrotron emission in the multi-component scenarios. A new era is now open, unveiling the physical processes and the dissipation mechanisms producing the prompt emission as well as the nature of the energy reservoirs powering GRB jets.