Based on a variational method adopting the dynamic pressure in the fluid and the acceleration in the solid as arguments in the functional, several substructure-subdomain methods are developed to describe the dynamical behaviour of fluid-structure systems excited externally. Formulations are presented of a displacement consistency model and a hybrid displacement model for the solid structure, a pressure equilibrium model for the fluid and a mixed substructure-subdomain model for the fluid-structure interacting system. These substructure-subdomain methods make the mixed finite-element approaches developed to analyse fluid-solid interactions more effective and efficient especially in calculating solutions to large complex engineering problems. This is achieved through the suitable selection of mode vectors to reduce the number of degrees of freedom accepted in the finite-element method to a manageable size without reducing significantly the accuracy of solution; by synthesis of the equations modelling the dynamic interactions and by developing techniques to eliminate mathematical difficulties occurring in the matrix formulations. By these means, consistent and unifying theoretical models are developed to describe the dynamical behaviour of the solid, fluid and their interactions which are in forms adaptable for solution on a personal computer. This is demonstrated by analysing a wide selection of fluid-structure (e.g. dam-water system excited by earthquake or explosion) and air-structure (e.g. structural-borne noise in a fuselage) interacting systems using purposely written computer software.