One of the many difficult tasks in requirements engineering for software-based systems is validation. The aspect of validation dealt with in this thesis is the justification that a proposed system, once implemented, will lead to the system in its environment displaying required behaviour. The work reported in this thesis is motivated by two problems with attempts at justification: . the need to reason about dependencies in composite systems . the need to carry out the re-justification of proposals as changes are made Solutions to both of these problems are found in the definition of a temporal logic and compositional reasoning method. These are based on the assumption/commitment approach to modelling, which was originally developed for reasoning about interference in concurrent systems. The approach is applied in this thesis in the area of requirements engineering through the provision of a highly modular style of object description in which: . the achievement of each object's commitme...