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Some observations are made concerning the process of developing complex systems. A broad class of systems, termed reactive, is singled out as being particularly problematic when it comes to finding satisfactory methods for behavioral description. In this paper we recommend the recently proposed statechart method for this purpose. Moreover, it is observed that most reactive systems cannot be developed in a linear stepwise fashion, but, rather, give rise to a two-dimensional development process, featuring behavioral aspects in the one dimension and implementational ones in the other. Concurrency may occur in both dimensions, as orthogonality of states in the one and as parallelism of subsystems in the other. A preliminary approach to working one's way through this" magic square" of system development is then presented. The ideas described herein seem to be relevant to a wide variety of application areas.