The strong cosmological motivations for measuring the CIBR have been extensively discussed in the references cited above, and are discussed further by Lonsdale at this Symposium. I therefore restrict my comments to providing some perspective on the observational search. In the past three decades while we have measured the properties of the CMBR in detail, we have only slowly learned something about what the CIBR isn't, but we still can't say what it is.
At the time of the discovery of the CMBR in the mid-1960's, there were no direct observational limits on cosmic electromagnetic radiation in the more than three decades from optical to millimeter wavelengths (for example, see Figure 6.3 in Peebles 1993). In particular, we could not by direct observations of the infrared sky brightness exclude the possibility that a radiant energy density as large as that required to close the Universe might be present in the spectral range from 1 µm to 1 cm! When the COBE proposals were written in 1974, a few pioneering rocket measurements in the infrared had limited that unlikely possibility, but one could not rule out a CIBR with an order of magnitude more integrated energy density than the CMBR, the dominant known contribution to the cosmic electromagnetic radiation. At the time of the COBE launch in 1989, though there had been more infrared instruments on rockets and the IRAS mission, there were still no definitive detections of a CIBR at any wavelength. Ressell & Turner (1990) compiled a comprehensive accounting of the diffuse photon background from radio to -ray energies. If the CIBR were in fact comparable to the measurements or upper limits compiled by Ressell & Turner, it was still possible that the CIBR might contain more cosmic energy density than any other part of the electromagnetic spectrum. Clearly, determining the cosmic infrared background is important for sharpening our knowledge of the radiant energy content of the Universe, as well as for providing clues to past and present astrophysical processes.