Corrosion damages is the biggest item on the maintenance bill of reinforced concrete (RC) structures. Chloride induced corrosion is the main cause for corrosion initiation. Chloride is the main cause for corrosion initiation in RC. Understanding the corrosion process and the parameters which influence the susceptibility for corrosion initiation, is essential for reducing the cost of corrosion in modern buildings. This book presents the theory of corrosion in reinforced concrete; and describe an investigation of the relationship between the micro-structure of the interfacial transition zone (ITZ) between the concrete and the embedded steel and the chloride concentration for corrosion initiation (chloride threshold). It includes a complete description of the method used in the investigation, results, and comprehensive discussion.