1. A vertical requirements development method for developing requirements for a system to be developed, the system to be developed having top-level requirements and being representable by a program specification tree made up of multiple layers of individual system elements, with each lower layer of the multiple layers comprising individual system elements having lower level requirements associated therewith compared to an upper layer, the method comprising the steps of: based on the top-level requirements, identifying a plurality of system level requirements analyses which, upon satisfaction, comply with the top-level requirements; and for each system level requirements analysis, creating a vertical flow down structure allocating specification requirements to each of the individual system elements that contribute to the satisfaction of that system level requirements analysis, regardless of the level of the individual system elements that contribute in the program specification tree.