The development and ultrastructure of cell walls of oil and mucilage cells in selected representatives of so-called primitive and derived dicotyledons are summarized, and compared with information on cell walls in other idioblasts and secretory or protective tissues. Oil and mucilage cells of Cinnamomum and Annona, and presumably other Laurales and Magnoliales, are both characterized by a suberized layer deposited against the primary wall. These taxa usually do not form mucilage or oil cavities through fusion of secretory cells. Hibiscus and other Malvales lack such a suberized layer in their mucilage cells and as a rule have mucilage cavities, resulting from the breakdown of common walls between mucilage cells. The inner, polysaccharide wall deposited against the suberized wall layer in oil cells strongly resembles the first deposited, dense mucilage layer in mucilage cells; the precise composition of these wall layers requires further study. It is hypothesized that, as in certain crystalliferous cells, laticifers, secretory trichomes, and epithelial cells of resin ducts, the suberized layer in oil and mucilage cells serves to compartmentalize the secretion product. In evolution the suberized layer may have been lost in mucilage cells in plant groups which possess exclusively mucilaginous secretory elements, and which are derived from ancestors with oil cells. However, an independent, de novo (parallel) origin of mucilage cells (and cavities) without suberized wall layers in derived and often unrelated dicotyledonous families may have been an alternative evolutionary pathway.