Reflection anisotropy spectroscopy (RAS) is a non-destructive optical probe of surfaces capable of operation within ultra-high vacuum and liquid environments. The RAS technique has been used to investigate the behaviour of surface electronic states on clean metal surfaces including energy shifts, depopulation and quantum confinement. RAS has been used to monitor the first stages of molecular assembly on metal surfaces in UHV and at the solid-liquid interface. Sensitivity to ad sorbate-surface bond formation and molecular orientation has been demonstrated. The Cu(110) and Au(110) surfaces have been the focus for RAS studies of molecular assembly in UHV and liquid environments, respectively. In this article, some aspects of the RA response of Cu(110) and Au(110) are described and the potential of RAS as a probe of molecular assembly at these surfaces is discussed. (C) 2003 Elsevier B.V. All rights reserved.