This book summarizes work done by the authors under the Esprit Tool Use project (1985-1990), at GMD in Karlsruhe and at Berlin University of Technology. It provides a comprehensive description of the generic development language Deva designed by the authors. Much of the research reported in this monograph is inspired by the work of Michel Sintzoff on formal program development; he contributed an enlightening Foreword. Deva is essentially a typed functional language with certain deduction rules. The difference with ordinary languages is, of course, the application domain: the types serve here to express propositions such as specifications or programs, rather than just data classes. Its practical applicability was tested on several non-trivial case studies. The whole book is written using the DVWEB system, a WEB for Deva, beeing implemented at the Berlin University of Technology.