  We develop a theory for filamentary molecular clouds including the effects of
ordered magnetic fields, and external pressure. We first derive a form of the
virial equation appropriate for filamentary clouds. By comparing with
observational results collected from the literature, we find that the fields
are likely helical. Secondly, we construct numerical, MHD models of filamentary
clouds that agree with the observational constraints. We find that our models
produce much more realistic density profiles $r\sim r^{-1.8 to -2}$ than
previous models, where the density falls off as $r^{-4}$.
