  We present a complete map of the Giant Molecular Clouds in M33 and show that
they lie exclusively on filaments of HI. The GMCs are localized on the extended
HI filaments, and HI filaments extend well beyond the radius where the GMCs are
found, suggesting that the H_2 forms from the HI and not vice-versa. We propose
that the fraction of neutral gas that is molecular at a particular radius is
determined primarily by the ambient hydrostatic pressure at that radius. This
leads to a prediction that the transition radius, the radius where the mean
surface density of HI and H_2 are equal, occurs at a constant value of the
stellar surface density. We show that for a sample of 26 galaxies, the stellar
surface density at the transition radius is constant to 40%.
