This chapter discusses the dynamical properties of multiple planet
systems. The orbits of these planets evolve due to tidal, resonant, and/or
secular (long-term) effects.  Basic analytical and numerical techniques
can describe these interactions. Multiple planet systems may also evolve
chaotically, and some principles of chaos theory are described. Finally,
this chapter discusses the current distributions of dynamical properties
of exoplanetary systems, possible origins of these distributions, and
compares exoplanetary systems to the Solar System.