The ability to rationally control surface topography of thin noble metal films has an enormous impact on the utility of such films for biosensor signal transduction. Nowhere is this more evident than in surface plasmon resonance (SPR), where changes in surface roughness of a few nanometers are easily detected. Accordingly, when immobilization of colloidal Au nanoparticles from solution is brought about by a biomolecular recognition event on an SPR substrate, dramatic changes in reflectivity are observed. This talk will focus on fundamental aspects of this phenomenon, including the dependence on colloidal Au particle size, particle coverage, and particle spacing, as well as on applications to ultrasensitive detection of proteins, nucleic acids, and small molecules.