The general notion of permeation capacity and its importance with respect to concrete durability has been introduced previously. In this section, some of the several measurements employed to quantify different aspects of permeation capacity in concretes are briefly discussed. These various techniques quantify the ability of a given concrete to transmit liquid water, water vapor, electrical current, or ions primarily through the pores within the concrete binder. Obviously this ability depends on the sizes of the pores, and on the degree to which the larger pores are effectively interconnected. A high degree of interconnectedness between the large pores is often referred to in the literature as `percolation’; its gradual elimination by progressive hydration constitutes `depercolation’. It is often suggested that in depercolation the larger pores are effectively isolated so that the only remaining connections between them are through `gel pores’. The present writer considers this most unlikely, in the light of his prior discussion of the concept of gel pores. The idea of progressively narrower `restricted interstices’, as diagrammed in Fig. 2.8, seems much more reasonable.