Various Galactic stellar spectroscopic surveys are currently underway, and each is expected to achieve high internal accuracy in terms of stellar parameters and abundances. A number of questions related to the formation and evolution of the Galaxy may be addressed based on samples of stars observed within each survey. In addition, complementary samples of stars may be constructed by combining data from different surveys. The Gaia FGK Benchmark Stars provide the necessary link to bring the quantities measured from different spectra with different methods onto the same scale. We selected 28 FGK stars and 5 M giants with available angular diameter θ, bolometric flux Fbol, and distance. We used the fundamental relation Teff ∝ F/θ0.5 to determine a reference effective temperature. We used the fundamental relation g ∝ M/R2, where M is the mass and R the radius, to determine a reference surface gravity. A homogeneous analysis of a high resolution and high signal-to-noise spectral library provides a reference metallicity (Blanco-Cuaresma et al. 2014A&A...566A..98B, Jofré et al. 2014A&A...564A.133J). These stars serve as a reference for testing and homogenizing large stellar surveys such as the Gaia mission and the Gaia-ESO public spectroscopic survey and for improving models of FGK-star atmospheres. A detailed discussion of the fundamental Teff and logg values will be presented in Heiter et al. (to be submitted).