Motivated by the Fermi gamma-ray space telescope results, in recent years
immense efforts were given to understanding the mechanism that leads to the
prompt emission observed. The failure of the optically thin emission models
(synchrotron and synchrotron self Compton) increased interest in alternative
models. Optically thick models, while having several advantages, also face
difficulty in capturing several key observables. Theoretical efforts are
focused in two main directions: (1) mechanisms that act to broaden the Planck
spectrum; and (2) combining the optically thin and optically thick models to a
hybrid model that could explain the key observables.Comment: 9 pages, 2 figures, 1 table; Invited review, to appear in the
proceedings of the Gamma-Ray Burst Symposium 2012- IAA-CSIC - Marbella,
editors: Castro-Tirado, A. J., Gorosabel, J. and Park, I.
