Complex systems require the use of an integrated and best balanced set of components. The integration and the balanced set are crucial issues, which require some sort of verifiable compositionality property of component  parts that contribute structurally, functionally, non functionally and interactionally to the total quality of the system design. This is even more important when dealing with the design of highly dependable systems. The concept of verifiable compositionality is much more demanding than the usual approach based on composition of building blocks. Moreover, the economy-driven whish to use COTS (Commercial Off the Shelf) and re-use of available components poses new further challenges. Integration, compositionality COTS and re-use appear to be the very challenging issues in the validation of complex, dependable systems used for controlling critical applications, and require a special effort towards the emergence of a new discipline - System Engineering - which will encompass and integrate the current design disciplines.