This monograph constitutes a major contribution to the development of secure electronic commerce. The book is based on the European R&D project SEMPER - Secure Electronic Marketplace for Europe which aims at securing electronic commerce as a whole by developing a technical security framework realized as a middleware. The first part of this monograph presents an introduction to electronic commerce in general and provides an overview of the proposed solutions; this part is accessible to everybody seriously interested in the topic and does not require a technical background except some basic familiarity with the Internet. The second part presents fundamentally new scientific and engineering results and sets the scene for future R&D activities in securing electronic commerce.