The laminins are a family of structural basement membrane components with major influences on cells. They are high molecular weight glycoproteins composed of three different but homologous chains, alpha, beta and gamma. At present 10 different chains have been identified. Each chain has a distinct structural organization of domains, some of which have been assigned biological activities, including self-assembly and interactions with other proteins. The particular importance of laminins for the formation and stability of cell adhesion complexes is highlighted in severe inherited diseases of muscle and skin. Merosin is the collective name for laminins that share a common subunit, the laminin alpha 2 chain. Merosin-deficient congenital muscular dystrophy (CMD) is caused by mutations in the laminin alpha 2 chain gene. The skin disease Herlitz junctional epidermolysis bullosa is caused by mutations in any of the laminin alpha 3, beta 3 or gamma 2 chain genes. The medical importance of laminins provides a further impetus to study the basic structure-function relationships in laminins in order to understand genotype-phenotype relationships and to design prenatal diagnostic tests and therapies aimed at compensating for specific defects.