The fundamental understanding of adsorption and self-organization of biological molecules at surfaces is of greatest importance for a huge variety of possible applications, ranging from molecular electronics to the study of biocompatible materials; hygiene, and biofouling. In spite of that, the characterization of the interactions of organic molecules of biological interest with surfaces is far from being complete. In the present paper we report on a combined microscopic (scanning tunneling microscopy (STM)) and spectroscopic (X-ray photoemission spectroscopy and high resolution electron energy loss spectroscopy) study of glutamic acid (Glu) adsorption and self-assembly on Ag(100) at different temperature. STM allows one to determine the structures of the Glu layers, for which empirical models are proposed, while photoemission spectra exclude adsorption in the zwitterionic form, which is the most common especially for weakly interacting substrates.