For the eighth edition we have made significant changes in both the structure and content to the previous version of this chapter concerning human leucocyte antigen (HLA) typing and antibody testing. This obviously reflects the continuing scientific and technical development in the field. DNA-based testing for HLA alleles now predominates over serological phenotyping and antibody detection/characterisation mostly involves non-cell-based methods. This HLA section is constructed of three main parts, concerning reagents (section 16.4), testing (sections 16.5, 16.6 and 16.7), and application to donor and patient investigations (section 16.8). For certain patient or donor investigations there is, of course, an overlap with the guidance given in this and the granulocyte or platelet immunology chapters (Chapters 17 and 18 respectively). This is particularly relevant to the laboratory investigations of platelet refractoriness and transfusion-related acute lung injury (TRALI), so diagrams are included (Figures 16.1 and 16.2) to indicate how the different guidelines relate to each other.