An advanced trajectory control system is defined for operation in the 1990's. Basic functions of trajectory control are the selection and control of the aircraft trajectory. For the 1990's, the system needs to operate on all available knowledge of the mission and environment. It needs to use this knowledge and the full maneuvering capabilities of the aircraft for responding to unexpected events during the mission, including control resource reconfiguration in the presence of failures. Decision, estimation, and control technologies must be integrated to provide these advanced capabilities. Identified as key to this integration are Artificial Intelligence and predictive path control technologies. These technologies and their potential application in trajectory control systems are discussed. Flight safety and system robustness are established as issues that need to be addressed early in the trajectory control system development.