This book constitutes the refereed proceedings of the 16th International Conference on Secure IT Systems, NordSec 2011, held in Tallinn, Estonia, October 26-28, 2011. The 16 revised papers presented together with 2 invited talks were carefully reviewed and selected from 51 submissions. The papers are organized in topical sections on applied cryptography, commercial security policies and their enforcement, communication and network security, security modeling and metrics, economics, law and social aspects of security, and software security and malware.