In this brief introductory paper the general structure and the molecular composition of the extracellular matrix are outlined. Ultrastructural morphology of the extracellular matrix is introduced and subsequently the molecular structure of each of the main protein families, which together make up the extracellular matrix, is reviewed. Collagens, laminins, tenascins, and proteoglycans are addressed. An important common feature is the domain structure of these in general very large proteins. Several families have domains in common, which favours extensive interactions. Integrins play an important role in these interactions and also in the communication between cells and the matrix. The extracellular matrix appears to be a very dynamic structure, which has a prominent role in normal development as well as in a variety of disease processes. Matrix metalloproteinases are essential actors in this complex interplay between cells and the extracellular matrix. Copyright 2003 John Wiley & Sons, Ltd.