The dynamic characteristics (i.e., natural frequencies and mode shapes) of a partially "lled and/or submerged, horizontal cylindrical shell are examined. In this investigation, it is assumed that the #uid is ideal, and #uid forces are associated with inertial e!ects only: namely, the #uid pressure on the wetted surface of the structure is in phase with the structural acceleration. The in vacuo dynamic characteristics of the cylindrical shell are obtained using standard "nite element software. In the &&wet'' part of the analysis, it is assumed that the shell structure preserves its in vacuo mode shapes when in contact with the contained and/or surrounding #uid and that each mode shape gives rise to a corresponding surface pressure distribution of the shell. The #uid}structure interaction e!ects are calculated in terms of generalized added masses, using a boundary integral equation method together with the method of images in order to impose an appropriate boundary condition on the free surface. To assess the in#uence of the contained and/or surrounding #uid on the dynamic behaviour of the shell structure, the wet natural frequencies and associated mode shapes were calculated and compared with available experimental measurements.