The millimeter waveband, between about 0.3 mm and 1 cm (Figure 7.1), has given us wonderful insights into the nature and distribution of interstellar matter, via the carbon monoxide spectral lines (section 5.7) and emission from warm dust grains (section 6.7). But by far its most important contribution to astronomy has been the study of the Cosmic Microwave Background radiation (CMB), a topic that earns its own chapter in this book. An additional reason for giving it a chapter to itself is that our understanding of the CMB draws almost equally on the techniques of radio astronomy and infrared astronomy—specifically the use of heterodyne-type radio receivers (which measure oscillating electric fields) and of bolometer-type infrared detectors (which collect and measure photon energies).