The major histocompatibility complex in swine (swine leucocyte antigen: SLA) is located on chromosome 7 with the class I and class III regions separated by the centromere from the class II region. The overall molecular organisation of the class I and III regions is well known, but further research is needed to establish that of the class II region. Approximately sixty genes have been characterised to date, including ten tightly packed SLA class I sequences. The exact number of functional polymorphic class I genes, as defined by serology, probably varies from one to four, depending on the haplotype. At least two other distantly class I-related gene families exist. The numerous and significant associations reported between SLA haplotypes and physiological traits are described. These traits include immune responsiveness to a variety of microbes and metazoan parasites, and male and female production and reproduction performance. The results obtained suggest that selection for specific SLA haplotypes may assist in the improvement of porcine production.