This chapter lays out the framework for understanding deterioration and deals with important concrete deterioration mechanisms. Concrete deterioration is a serious problem for the durability of many concrete structures. Premature deterioration of concrete structures has severe consequences, such as threat to economic growth, depletion of natural and non-renewable resources, impacts on human safety, and costs. Deterioration mechanisms such as chemical attack, leaching, chloride ingress, or carbonation relate to the ease with which fluids or ions can move through the concrete microstructure. Permeation is the movement of fluids through the pore structure under a pressure gradient, with the pores saturated with the particular fluid. Absorption occurs when a fluid is drawn into a porous unsaturated material due to capillary suction, and depends on the pore geometry and the degree of saturation. Migration is the movement of ions in a solution under an electrical field, typically used in accelerated chloride tests.