Nonlinear impact of perturbation theory on numerical relativity

I discuss the impact of gauge-invariant perturbation theory, as developed originally by Vincent Moncrief, on numerical simulations of Einstein's theory. Far from being replaced by numerical relativity, perturbative approaches remain essential for analysing, interpreting, extending and complementing fully nonlinear approaches. In the last decade, as computers have become ever more powerful tools for studying the full nonlinear equations, the power and application of perturbation theory has also grown. Its impact on numerical relativity is profound (decidedly nonlinear), and will surely continue to be for years to come.