To develop plans for a viable ATC system over the next 25 years a whole spectrum of studies can be conducted, each concerned with a different time frame. The spectrum, when laid out over time, is bracketed by two extreme cases. 1. One extreme is analysis of the present ATC system to identify its shortcomings, followed by synthesis studies to identify evolutionary ways of overcoming these shortcomings. 2. At the other extreme one can study the ATC system sufficiently far into the future that decisions need not be constrained by existing equipment, airspace utilization and procedures. Between these two extremes are other studies concerned with developing plans for intermediate time frames. To be effective, study (1) must be done immediately. Study (2) should precede many of the studies for intermediate time frames since the results of study (2) should be available to influence what is done in intervening periods. In this report we view the Fourth Generation Concept Formulation Study as study (2). Thus the results are not strongly influenced by present day equipment and are influenced by present airspace utilization and procedures only where they appear to be as good or better than other ways of operating the system. The ATC system is designed to fulfill certain needs of the nation. To satisfy those needs the ATC system must achieve specific objectives. The major objective of the system is to provide safe, expeditious flow of air traffic at reasonable cost. It is generally accepted that to achieve this objective certain functions in the area of surveillance, navigation, and communication must be performed and that considerable data processing in the ATC system is required. The examination of ways of achieving various performance levels of these functions is the subject of concept formulation in -- the areas of surveillance, navigation, communication and data processing. Given that the surveillance, communication, and navigation functions are performed, there are other functions which are required in order to achieve the objectives of the ATC system. These functions, which include flow control, metering, sequencing, spacing, conformance and hazard monitoring, and conflict and hazard resolution make up the control aspects of the ATC system. In terms of the operation of the ATC system the surveillance, communication and navigation functions must be performed if the control functions are to be performed. In terms of the design of the system, however, the surveillance, communication, and navigation functions cannot be specified in detail until the required control functions are determined in detail. Thus, studies in the control area must be performed in a timely manner in order to insure that studies in the other areas will be conducted at a high level of efficiency. Control studies seek to determine the detailed characteristics of the functions which will be performed to achieve the objectives of the ATC system.