This book constitutes the refereed proceedings of the 12th International Symposium on Stabilization, Safety, and Security of Distributed Systems, SSS 2010, held in New York, USA, in September 2010. The 39 revised full papers were carefully reviewed and selected from 90 submissions. The papers address all safety and security-related aspects of self-stabilizing systems in various areas. The most topics related to self-* systems. The tracks were: self-stabilization; self-organization; ad-hoc, sensor, and dynamic networks; peer to peer; fault-tolerance and dependable systems; safety and verification; swarm, amorphous, spatial, and complex systems; security; cryptography, and discrete distributed algorithms.