The connective tissues are crucial for multicellular organisms, playing many diverse roles both structurally and mechanically as well as providing a protective function. They make up the frame of the mammalian body and add strength and elasticity, while their protective function is important both on the outside of the body and inside, where they surround and support the inner organs. Connective tissues consist mainly of extracellular matrix (ECM) surrounding a sparse population of cells. The framework of the ECM consists of insoluble fibrils primarily of collagen. The structure of the fibrils varies depending on the demands the tissue has to meet. An important determinant is the proportion of different collagen types. However, the shape, organization and mechanical properties of the fibrils also depend on the presence of other ECM glycoproteins and proteoglycans. Cartilage represents a specialized connective tissue that is a major target in joint diseases affecting more than 10% of the population. To understand the biology and the pathology of events in the articular cartilage in repair and in disease leading to destruction, it is of essence to discern key constituents in the matrix and their functions. Most of the macromolecules in the cartilage are not unique to this tissue and are present in other tissues where they have similar functions. Thus the more general discussion on the constituents of the fibrillar networks very much applies to cartilage, where it contains a mixture of molecules specific to cartilage and others present in many tissues. The major constituents of cartilage are collagen type II, hyaluronan and the cartilage-specific proteoglycan aggrecan. Aggrecan binds to hyaluronan to form large aggregates, which due to their extensive charge, hydrate the tissue resulting in a high osmotic swelling pressure. Collagen type II is by far the major constituent of a network of thin fibrils entrapping and interacting with the aggrecan/ hyaluronan aggregates thus endowing the cartilage with its ability to resist compressive load and providing both mechanical stiffness and resilience. The bulk collagen in banded fibrils in almost all tissues is type I collagen. A notable exception from this rule is hyaline cartilage, where type II collagen replaces type I collagen. However, the mode of aggregation of these collagens is to a significant extent determined by other matrix macromolecules with which they are alloyed into D-periodically banded fibrils. In this way, structural and functional tissue