For the course ADVANCED TOPICS ON ANALYSIS AND DESIGN OF CRITICAL SYSTEMS i attended in Politecnico di Milano in 2007, I had to study and present the Neil Immerman proof of NSPACE = CO-NSPACE. This is the small presentation i prepared with Simone Campanoni. We present the two available proofs Szelepcsènyi’s and Immerman’s. I hope this will make someone life easier.