This paper presents the importance of microstructural studies for understanding concrete durability. Three examples-sulfate attack; alkali silica reaction and bacteriogenic corrosion in sewers arc used to illustrate the pitfalls which can arise from conventional empirical testing, particularly when blended or non-Portland cements are used. The pressure to provide more sustainable cements with lower CO, emissions will inevitably lead to major changes in the chemistry of concrete. Therefore it is important to properly understand durability and the microstructural mechanisms involved to design appropriate performance based criteria to using the increasing diversity of concrete which will become available.