To deal with general loading conditions, it is necessary to include bending and torsion into the equilibrium equations of arches and shells; which leads to interesting coupling effects between various elementary modes of deformation. In the case of arches, in-plane bending is found to be coupled with tangential stretching and out-of-plane bending with torsion. In the case of shells, all the elementary modes of deformation are found to be coupled together, except in a few particular loading cases. The problem of shell vibrations was first attacked by Sophie Germain in the early nineteenth century. However, the basic development of the thin shell theory is due to Love in 1888. As already indicated in the preceding chapter, Love s model is based on simplifying assumptions which extend in a natural manner those already used to model straight beams and plates. The thin shell theory was the object of various refinements during the twentieth century. As a result, there exists a wide variety of shell equations. However, all of them are basically of the Love type, differing only by the approximations made to deal with the metric coefficients G _?, G _?. Furthermore, such differences turn out to be of little practical importance. Therefore, presentation is restricted here to the Love model. Particularization to cylindrical shells of revolution gives us the opportunity to discuss a few problems of practical interest and the validity of various simplifications of Love s equations, in relation to the specificities of the loading considered.