Multibody dynamics is the study of the motion of assemblies of bodies, rigid or flexible, that undergo large motion in the 3D space. The free motion of bodies is restrained by joints. Every joint links two bodies in two points. These joints are idealizations of the contact between the two bodies. Joints are characterized by the motion that they allow between the two bodies that they connect. For example, a revolute joint allows one relative rotation between two bodies, constrains all three relative translations, and blocks the two other relative rotations.

The primary unknowns of a multibody dynamics solution are the translation and rotation of each body and the motion in the joints themselves. The output quantities on rigid bodies are the forces that develop in the joints and flow through the rigid bodies, as opposed to a structural analysis where the output quantities are strains or stresses. Flexible bodies can be included in a multibody analysis. These flexible bodies will have both joint forces and stress and strain results.