Much has been written on the subject of systems design, which can be treated in several ways. Some choose to go in for deep mathematical analysis to solve the problem, some for management techniques, but the object of this paper is to relate our observations to the basic logic and practice that we actually use in aircraft systems design, which includes both techniques where applicable, together with a deep experience continued from one aircraft to the next, which is a most essential factor in achieving the standard that is required. The subject is so wide that it will be impossible to deal in detail with the many aspects, but we hope to cover sufficient of the philosophy to provide an appreciation of the task. The paper does, however, attempt to emphasise that unique skill in aircraft design, i.e., airframe systems engineering.