Abstract This chapter describes how the characteristics of the pore structure of cement-based materials play a fundamental role in governing the transport processes in concrete, and hence, its durability. Variations in the size of the pores in concrete and their influence on both transport properties and durability are highlighted in this chapter. Numerous test methods can be used to characterise the pore structure and permeability of concrete. Environmental factors, such as temperature and moisture variations, affect all transport processes, and hence, there is a need to consider their effects on the measurement of transport properties. Both laboratory and field data highlight the dependence of durability on transport properties. The properties of the constituent materials, the methods of manufacturing concrete and the subsequent treatment of concrete affect both the pore structure and the transport processes, and hence, to design durable structures, attention should be given to these factors.