The subject of this chapter is the dynamics of closed-chain multibody systems. These are systems whose underlying digraphs are not trees, but may be multiply- connected and include directed cycles, as illustrated in Fig. 8.6 on page 152. Section 9.6.2 on page 185 discussed some of the hurdles in extending SKOmodels to closed- chain systems. The issues identified included the fact that the SKO operator is not nilpotent when the digraph has directed cycles, and that the hinge generalized coor- dinates are not independent when there are multiply-connected nodes in the system digraph.