We present a study of the InterStellar Medium in the outer parts of spiral galaxies. The observational part includes new observations (detections) of molecular gas far out in spiral galaxies, coupled with existing maps of the HI, UV, and dust continuum emission. Modelling of the stellar UV-NIR emission including absorption by dust grains provides a picture of the intrinsic radiation fields. These models are used to calculate grain temperatures which are then compared with observations of the dust emission. Photo-ionisation codes are then used to estimate the properties of the various phases of the outer disk ISM. The models and how they compare to observations of a variety of spirals from low to normal metallicity and luminosity will be the heart of the presentation. The smaller objects, like M33, are likely good surrogates for intermediate redshift objects which are smaller, bluer, and probably slightly less metallic than today's large spirals. The goal is to provide reliable estimates of the amount of mass in each of the phases (H2, HI, and HII) in the outer disks of these galaxies, which can reasonably be assumed to be representative. The star formation efficiency (star formation rate divided by available gas mass) can then be compared over a much broader range of radii than up to now in order to understand how galaxies form stars (i.e. the gas-stars cycle).Read less <