This project involves the stability analysis of two problems: one in collisionless stellar dynamics and the other in incompressible fluid flows. In the former case, the energy- Casimir method will be used to examine the stability properties of non-spherical galactic models. In the latter case, ideas form Hamiltonian systems and symmetries will be used to examine the nonlinear stability of Kida's periodic elliptic vortices in a planar straining flows, with applications to a study of three- dimensional vortex structures in the oceans.