The security of computer networks plays a strategic role in modern computer systems. In order to enforce high protection levels against threats, a number of software tools have been currently developed. Intrusion Detection Systems aim at detecting intruders who elude "first line" protection. In this paper, a pattern recognition approach to network intrusion detection based on the fusion of multiple classifiers is proposed. Five decision fusion methods are assessed by experiments and their performances compared. The potentialities of classifier fusion for the development of effective intrusion detection systems are evaluated and discussed.