An experimental study of the vibrations of cantilever plates in air and in water has been conducted, and the results are compared with theoretical predictions based on simple beam theory and thin-plate theory. Both theories are modified to include an apparent mass factor which is derived from chordwise hydrodynamic strip theory. Good theoretical and experimental correlation is achieved upon further including an empirical correction factor which allows for the influence of plate aspect ratio and thickness ratio on the results. Liquid free surface and partial submergence effects are determined experimentally.