Essentials. -- Introduction. -- Single degree-of-freedom systems. -- Systems with more than one degree of freedom. -- Continuous systems. -- Finite-element vibration analysis. -- Group-theoretic formulations. -- Basic concepts of symmetry groups and representation theory. -- Rectilimear models. -- Plane structural grids. -- High-tension cable nets. -- Tinite-difference formulations for plates. -- Finite-element formulations for symmetric elements.