We propose to obtain the first complete, fully-sampled map of a
 nearby prototypical Giant Molecular Cloud (GMC) - either W3 (spring
 launch) or NGC 7538 (fall launch) - at wavelengths of 135 and 200
 microns, corresponding to the expected peak in the spectral energy
 distributions (SEDs) of these objects. These data will be combined
 with our existing optical and infrared images, IRAS ISSA and HIRES
 images, proposed submillimeter (350 - 1300 micron) maps, and radio
 continuum maps to provide the first complete picture of the SED
 within a GMC on spatial scales of ~1 arcmin (~0.6 pc). Existing
 data indicate that most of the molecular gas in GMCs (>80%) is not
 directly associated with the more widely studied, warm, massive
 OB star-forming cores. Over most of the projected area of a GMC,
 both the ambient material and the lower mass star-forming cores
 appear to emit the bulk of their luminosity in the 100 - 250 micron
 band. Thus our ISO data will allow us, for the first time, to
 carry out a complete census of the true distribution of luminosity
 sources throughout a GMC, and to make greatly improved estimates
 of the dust temperature and heating and cooling rates, which can
 then be used to construct more realistic models for the origin and
 evolution of GMCs.