  The origin of gamma-ray burst (GRB) prompt emission, bursts of gamma-rays
lasting from shorter than one second to thousands of seconds, remains not fully
understood after more than 40 years of observations. The uncertainties lie in
several open questions in the GRB physics, including jet composition, energy
dissipation mechanism, particle acceleration mechanism, and radiation
mechanism. Recent broad-band observations of prompt emission with Fermi sharpen
the debates in these areas, which stimulated intense theoretical investigations
invoking very different ideas. I will review these debates, and argue that the
current data suggest the following picture: A quasi-thermal spectral component
originating from the photosphere of the relativistic ejecta has been detected
in some GRBs. Even though in some cases (e.g. GRB 090902B) this component
dominates the spectrum, in most GRBs, this component either forms a
sub-dominant "shoulder" spectral component in the low energy spectral regime of
the more dominant "Band" component, or is not detectable at all. The main
"Band" spectral component likely originates from the optically thin region due
to synchrotron radiation. The diverse magnetization in the GRB central engine
is likely the origin of the observed diverse prompt emission properties among
bursts.
