Several applications and adaptations of multivariable control theory for aircraft control law developments are described. Applications include solution of a lateral oscillation problem in the Boeing 767 autopilot, a longitudinal control law giving excellent handling qualities for stable or unstable airframes, and a high-agility fighter giving superior flying qualities up to 90 degrees angle of attack. Adaptations of control theory include meeting the Gibson longitudinal flying qualities criterion, a unified application of linear quadratic design theory, and application of the structured singular value. The paper also includes a discussion of the Boeing software tools used for model building, control analysis and design.