Chemical selectivity of biosensors is derived from biological materials interfaced to the surface of transducing devices. Molecular recognition events lead to macroscopic function suitable for analytical measurements. The structure-function relationships of biochemical species at interfaces must be established to characterize and optimize biosensor operation. The techniques of ellipsometry, fluorescence microscopy, electron microscopy, and scanning tunneling microscopy are used to investigate the structure of monolayers and multilayers of proteins and lipids at interfaces that are prepared by Langmuir-Blodgett techniques and by self-assembly from bulk solution. The relative merits and limitations of the measurement techniques in the determination of aspects of interfacial structure are considered.