The document summarizes key concepts from lectures on multi-degree of freedom structural dynamics. It discusses: 1) Modeling structures with multiple masses connected by springs as a multi-degree of freedom system and deriving equations of motion. 2) Solving the equations of motion for free and forced vibrations using modal analysis to transform the coupled equations into uncoupled single-degree of freedom systems. 3) Applying the analysis to study the cross-wind response of slender towers subjected to vortex shedding forces, modeling the forces as sinusoidal inputs.