Durability is defined as the ability of a material, component, or structure to remain serviceable for a desired period within its design environment. Serviceability refers to the ability of a material, component, or structure to fulfil its design function, for example to retain water. A required property of concrete is long-term durability, and here aggregates also have a role. This chapter focuses on that role. Concrete is regarded as a 'durable' material, and this is generally true provided care is taken in the design of the material and precautions are exercised when it is required to function in particularly harsh or aggressive environments. Aggregates make a substantial contribution to concrete durability by normally being durable, hard, and longwearing. Aggregates in a concrete matrix reduce the cementitious paste component which is susceptible to deterioration, being more thermo-dynamically unstable. Aggregates are more resistant to external aggressive attack and are usually less permeable than the matrix. However, certain aggregates may exhibit properties that render them susceptible to deterioration and thus compromise concrete durability. In other cases, aggregates may be susceptible to attack from external agents.