  Crystalline acetanilide is a an organic solid with peptide bond structure
similar to that of proteins. Two states appear in the amide I spectral region
having drastically different properties: one is strongly temperature dependent
and disappears at high temperatures while the other is stable at all
temperatures. Experimental and theoretical work over the past twenty five years
has assigned the former to a selftrapped state while the latter to an extended
free exciton state. In this article we review the experimental and theoretical
developments on acetanilide paying particular attention to issues that are
still pending. Although the interpretation of the states is experimentally
sound, we find that specific theoretical comprehension is still lacking. Among
the issues that that appear not well understood is the effective dimensionality
of the selftrapped polaron and free exciton states.
