Hubble’s astronomical observations and Lemaître’s theoretical considerations suggested that our universe is expanding, therefore it was reasonable to assume that it had emerged from a state of very small extent and very high density. As early as the 1940s, it was speculated that the radiation from this period could still should be detectable today. The exact analysis of the spectrum of this microwave background radiation over the past few decades has fundamentally expanded our knowledge about the universe. In this chapter, we discuss what information can be gained from these analyses about the young universe.