One of the greatest challenges to engineers designing concrete structures and infrastructure is that of durability: a fundamental objective of design should be that a structure is able to satisfy its functions for the duration of its intended working life. This chapter focuses on biodeterioration processes directly caused by living organisms in close proximity to concrete. The types of organism which can take part in the deterioration of concrete are surprisingly varied, including bacteria, fungi, both higher and lower plants, and a possibly surprising array of animals including molluscs and marine worms. The mechanisms of biodeterioration of concrete include both chemical processes—such as leaching of cement resulting from the formation of acidic substances—and physical ones—for instance, from pressures exerted by root growth. Deterioration mechanisms can also involve fairly complex combinations of both. Deterioration rates vary considerably and depend on the type and number of organisms involved, as well as the environmental conditions and the composition and properties of the concrete.