  The estimation of molecular abundances in interstellar clouds from
spectroscopic observations requires radiative transfer calculations, which
depend on basic molecular input data. This paper reviews recent developments in
the fields of molecular data and radiative transfer. The first part is an
overview of radiative transfer techniques, along with a "road map" showing
which technique should be used in which situation. The second part is a review
of measurements and calculations of molecular spectroscopic and collisional
data, with a summary of recent collisional calculations and suggested modeling
strategies if collision data are unavailable. The paper concludes with an
overview of future developments and needs in the areas of radiative transfer
and molecular data.
