In this article we summarise on-going work on the so-called Gaia FGK Benchmark Stars. This work consists of the determination of their atmospheric parameters and of the construction of a high-resolution spectral library. The definition of such a set of reference stars has become crucial in the current era of large spectroscopic surveys. Only with homogeneous and well documented stellar parameters can one exploit these surveys consistently and understand the structure and history of the Milky Way and therefore other of galaxies in the Universe.