This section will define the interfacial transition zone (ITZ) as the region around the aggregate particles in concrete where the cement paste has different characteristics than the bulk paste. The explanation will cover how the ITZ is formed during the mixing and setting of concrete, influenced by factors such as water migration toward aggregates and variations in particle packing. The mechanisms by which the ITZ affects the microstructure of concrete, including the density and continuity of the cement matrix, will be detailed.