Chloride ions have been identified as an important cause of the initiation and acceleration of the corrosion of the reinforcements within modern concrete structures. This book introduces a comprehensive mathematical model used to describe the mass transport behavior of ionic species in porous media. It takes account most of the major mass transport mechanisms that involved. They are the convection with pore solution, the diffusion in pore solution, the physical/chemical adsorption/desorption on/off the solid phase of porous media and the dynamic electrochemical interactions within the local electrical field generated. The developed mathematical model has subsequently been adopted to model the chloride ingress process in the concrete under saline environmental conditions for the purpose of the service life assessment and the chloride removal process of the concrete under electrochemical treatment for concrete rehabilitation. This mathematical model and numerical methods described in this book can also be used for other environmental problems and engineering applications, such as underground pollutant disperse and land contamination modelling.