It is now time to turn to issues of performance. As noted in earlier chapters, performance specifications typically involve the closed-loop relations between the exogenous inputs $$w$$ and the regulated outputs $$z$$. These relationships are typically captured through the use of the signal and system norms. The analysis of a given controlled system usually involves evaluating the appropriate norms. The synthesis of a controller is a harder problem, as it involves picking a feedback compensator $$K$$ for which the closed-loop performance specifications are attained.