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Laplace-Lagrange secular theory (or Laplace-Lagrange secular evolution theory) is a theory of the (secular) perturbation of planet orbits (a perturbation theory) when two or more astronomical objects are orbiting a third, that grew from the work of Laplace and Lagrange. It was developed to investigate whether Newtonian dynamics is consistent with stable solar system planetary orbits, i.e., is it consistent with known observation and with theories of the age and evolution of the solar system. Today the theory can be compared/validated with long-term computer orbit simulations. The theory has naturally gained recent interest for analogous analysis of orbits of other systems such as binary stars and systems including extra-solar planets.