  Here, we present a set of time-dependent 3D RMHD simulations of a M-dwarf
star representative of AD Leo, which extend from the upper convection zone into
the chromosphere. The 3D model atmospheres are characterized by a very dynamic
and intermittent structure on small spatial and temporal scales and a wealth of
physical processes, which by nature cannot be described by means of 1D static
model atmospheres. Artificial observations of these models imply that a
combination of complementary diagnostics such as Ca II lines and the continuum
intensity from UV to millimeter wavelengths, probe various properties of the
dynamics, thermal and magnetic structure of the photosphere and the
chromosphere and thus provide measures of stellar activity, which can be
compared to observations. The complicated magnetic field structure and its
imprint in synthetic diagnostics may have important implications for the
understanding and characterization of stellar activity and with it possibly for
the evaluation of planetary habitability around active M-dwarf stars.
