  X-ray observations are a direct diagnostic of fast electrons produced in
solar flares, energized during the energy release process and directed towards
the Sun. Since the properties of accelerated electrons can be substantially
changed during their transport and interaction with the background plasma, a
model must ultimately be applied to X-ray observations in order to understand
the mechanism responsible for their acceleration. A cold thick target model is
ubiquitously used for this task, since it provides a simple analytic
relationship between the accelerated electron spectrum and the emitting
electron spectrum in the X-ray source, with the latter quantity readily
obtained from X-ray observations. However, such a model is inappropriate for
the majority of solar flares in which the electrons propagate in a hot
megaKelvin plasma, because it does not take into account the physics of
thermalization of fast electrons. The use of a more realistic model, properly
accounting for the properties of the background plasma, and the collisional
diffusion and thermalization of electrons, can alleviate or even remove many of
the traditional problems associated with the cold thick target model and the
deduction of the accelerated electron spectrum from X-ray spectroscopy, such as
the number problem and the need to impose an ad hoc low energy cut-off.
