  The shocks of several young supernova remnants (SNR) are often associated
with very thin optical filaments dominated by Balmer emission resulting from
charge-exchange and collisional excitation between neutral Hydrogen from the
interstellar medium and shocked protons and electrons. Optical lines are a
direct probe of the conditions at the shock, in particular the width of the
narrow and broad components reflect the temperature upstream and downstream of
the shock, respectively. When the shock accelerate efficiently non-thermal
particles, the shock structure changes producing anomalous Balmer lines and it
is possible to use their line shape and their spatial profile to check the
efficiency of SNR shocks in accelerating cosmic rays. Here we illustrate the
kinetic theory of shock acceleration in presence of neutrals with some
applications to young SNRs. We show that in three cases (RCW 86, SNR 0509-67.5
and Tycho) anomalous Balmer lines can be explained assuming that a fraction of
$\sim 10\%$ of the total shock kinetic energy is converted into not thermal
particles, while in one single case, the northwestern part of SN 1006, there is
no evidence of efficient acceleration.
