The best-known protein components of the plant cell wall have highly repetitive, proline-rich sequences. The use of recombinant DNA approaches has enabled complete sequences of these proteins to be determined and features of the expression of the corresponding genes to be examined. These results, coupled with the use of immunological techniques, have shown that proline-rich proteins are interesting probes to study developmental and defence processes in plants. In this review, the sequence and expression of different groups of proline-rich proteins in plants are presented. These groups include hydroxyproline-rich glycoproteins (HRGP) or extensins, proline-rich proteins (PRP) and glycine-rich proteins (GRP). The specific features of each group and the possible functions of these proteins are discussed, as well as the data available on the mechanisms controlling the expression of their corresponding genes.