This paper reports recent advances in ellipsometric characterization of silver and gold nanoparticles (NPs). Three systems are investigated: films composed of Au NPs distributed in shape, films with a gradient of concentration of Ag NPs, and colloidal suspensions composed of Au NP chains. The authors show that ellipsometry is highly sensitive to the plasmon resonance of NPs. This later can be used as an efficient probe of the NP morphology. The ellipsometric results are systematically compared to those that are obtained by transmission electron microscopy. The authors demonstrate that quantitative insights such as the shape distribution, the concentration, and the organization of NPs can be extracted from ellipsometric spectra.