This book contains the combined treatment of several problems of control systems theory, such as the HINFINITY control problem, the Nehari problem and robust stabilisation. These topics are described from a new perspective which is essentially created by an original generalisation of the algebraic Riccati theory to the indefinite sign case. The theory is developed using methods including the Popov function, the Kalman-Popov-Yakubovich system in J-form, and the extended Hamiltonian pencil. The signature condition on the Popov function plays a crucial role in providing the unified approach to solving the control problems considered. Particular attention is paid to the optimal solutions of the HINFINITY control problem and the Nehari problem for which a singular perturbation-based technique is employed to derive explicit well-conditioned computational formulae. Numerical examples, mainly from aeronautics, illustrate the performances of the proposed procedures and design algorithms. Audience: This volume will be of interest to researchers, graduate students and control engineers working in systems and control theory, mathematical systems theory, optimal control, aerospace engineering and numerical analysis.