  Significant populations of field L and T dwarfs are now known, and we
anticipate the discovery of even cooler dwarfs by Spitzer and ground-based
infrared surveys. However, as the number of known L and T dwarfs increases so
does the range in their observational properties, and difficulties have arisen
in interpreting the observations. Although modellers have made significant
advances, the complexity of the very low temperature, high pressure,
photospheres means that problems remain such as the treatment of grain
condensation as well as incomplete and non-equilibrium molecular chemistry.
Also, there are several parameters which control the observed spectral energy
distribution - effective temperature, grain sedimentation efficiency,
metallicity and gravity - and their effects are not well understood. In this
paper, based on a splinter session, we discuss classification schemes for L and
T dwarfs, their dependency on wavelength, and the effects of the parameters
T_eff, f_sed, [m/H] and log g on optical and infrared spectra. We will discuss
the various hypotheses that have been presented for the transition from the
dusty L types to the clear atmosphere T types. We conclude with a brief
discussion of the spectral class beyond T. Authors of each Section are
identified by their initials.
