  Observational facilities allow now the detection of optical and IR spectra of
young M- and L-dwarfs. This enables empirical comparisons with old M- and L-
dwarfs, and detailed studies in comparison with synthetic spectra. While
classical stellar atmosphere physics seems perfectly appropriate for old
M-dwarfs, more physical and chemical processes, cloud formation in particular,
needs to be modelled in the substellar regime to allow a detailed spectral
interpretation.
  Not much is known so far about the details of the inset of cloud formation at
the spectral transition region between M and L dwarfs. Furthermore there is
observational evidence for diversity in the dust properties of objects having
the same spectral type. Do we understand these differences? The question is
also how young M- and L-dwarfs need to be classified, which stellar parameter
do they have and whether degenerations in the stellar parameter space due to
the changing atmosphere physics are present, like in the L-T transition region.
  The Splinter was driven by these questions which we will use to encourage
interactions between observation and theory. Given the recent advances, both in
observations and spectral modelling, an intensive discussion between observers
and theoreticians will create new synergies in our field.
