This book summarizes work done by the authors under theEsprit Tool Use project (1985-1990), at GMD in Karlsruhe andat Berlin University of Technology. It provides acomprehensive description of the generic developmentlanguage Deva designed by the authors. Much of the researchreported in this monograph is inspired by the work of MichelSintzoff on formal program development; he contributed anenlightening Foreword.Deva is essentially a typed functional language with certaindeduction rules. The difference with ordinary languages is,of course, the application domain: the types serve here toexpress propositions such as specifications or programs,rather than just data classes. Its practical applicabilitywas tested on several non-trivial case studies. The wholebook is written using the DVWEB system, a WEB for Deva,beeing implemented at the Berlin University of Technology.