Proteoglycans ( PGs) have, until recently, been poorly characterized at the amino acid sequence level. Since 1985, when the first proteoglycan core protein structure was determined 1 11, structural information on these complex molecules has become relatively plentiful. Proteoglycans are a good example of protein construction through the assembly of domains. It is tempting to assume that the individual domains define discrete functions: this will be discussed here. The major proteoglycans of cartilage are the large, aggregating chondroitin sulphate proteoglycans and the small, dermatan/chondroitin sulphate PGs (PG-S 1 and PG-S2). The protein cores of these PGs have recently been sequenced. The sequence of the large PG from rat chondrosarcoma was determined by Doege el ul. 121 and the sequences of small PGs were determined by Krusius & Ruoslahti (PG-S2 from human skin) [ 31, Day et al. (PG-S2 from bovine bone) j4J, Fisher et ul. (PG-SI from human bone) [ S j and Nearne et ul. (PG-SI from bovine cartilage) 16). All of these molecules have discrete domains which are amenable to theoretical analysis of their structure and function. In some cases, it has been possible to analyse their function experimentally and examine thcir structure at a gross level by rotary shadowing.