The cosmic microwave background (CMB) radiation, first light released in the universe 380,000 years after the Big Bang, is still the most perfect blackbody radiation ever measured in nature, with a temperature of T=2.725 K. Yet, several mechanisms of interaction between matter and radiation throughout the history of the universe transfer energy to the CMB photons, thus causing tiny deviations from a perfect blackbody (smaller than 10 part in one million), commonly referred to as CMB spectral distortions. I will discuss about various types of CMB spectral distortions and show how spectral distortions to the CMB blackbody radiation provide a unique tool to probe the early Universe and constrain cosmology.