This Technical Report gives the designations, descriptions and reference micrographs of the matrix structures of cast irons.It applies to the following types of cast irons:— grey cast irons (Table 4.1);— spheroidal graphite cast irons (Table 4.2);— austenitic cast irons (Table 4.3);— malleable cast irons (Table 4.4);— compacted (vermicular) graphite cast irons (Table 4.5);— ausferritic spheroidal graphite cast irons (Table 4.6); — abrasion-resistant cast irons (Table 4.7).Each matrix structure is defined with explanations and micrographs.Unless otherwise stated in Clause 4, the micrographs shown correspond to samples etched with a solution of 2 % nitric acid in ethanol (Nital).