Cartesiana is a system to support the construction of software on the basis of formal methods. It is currently being used for the development of a provably secure system in a pilot project in West Germany. The quality criteria applied go beyond A1 and include program level verification. The Cartesiana approach to meet these criteria emphasizes constructive techniques. Proof rules are used to derive a solution in such a way that development and verification are closely combined. Rules and specified formal requirements guide the development process. Some of the methodological aspects are illustrated by way of example. A concise overview over the main functionalities of the system is given.>