Requirements defects remain a significant problem in the development of all software intensive systems including information systems. Progress with this fundamental problem is possible once we recognize that individual functional requirements represent fragments of behavior, while a design that satisfies a set of functional requirements represents integrated behavior. This perspective admits the prospect of constructing a design out of its requirements. A formal representation for individual functional requirements, called behavior trees makes this possible. Behavior trees of individual functional requirements may be composed, one at a time, to create an integrated design behaviour tree (DBT). Different classes of defects are detected at each stage of the development process. Defects may be found at the translation of behaviour trees, and then at the integration of behaviour trees and when individual component behaviour trees are projected from the DBT. Other defects may be found by inspection and model-checking of the DBT.