If the microwave background radiation discovered by Penzias and Wilson is a relic of the Hot Big Bang origin of the Universe, it ought to have a thermal spectrum. The first few measurements of the spectrum of the radiation were consistent with a Planck curve with T0 = 3 K, hence strengthening our belief that the microwave background was indeed cosmic in origin. In the more than 25 years that have followed the early measurements of Penzias and Wilson (1965, 1967), Roll and Wilkinson (1966), Howell and Shakeshaft (1966), Field and Hitchcock (1966) and Thaddeus and Clauser (1966), a number of increasingly precise measurements of the spectrum have been made. The range of wave-lengths has been extended to 75 cm ≳ λ ≳0.5 mm. An early aim of these observational programs was to check the Big Bang model for the origin of the microwave radiation; another was to look for small perturbations in the spectrum of the radiation, which might have been produced by energetic processes occurring well after the Big Bang. These processes, the nature of the spectral perturbations they produce, and the limits the observations place on them, will all be discussed in Chapter 5. In this chapter, we will look at the observational techniques and the resulting values of T0 found at different wavelengths.