Many of the invertebrates possess calcium carbonate spicules. This paper is a review of the formation of these structures in the Porifera, Coelenterata, Platyhelminthes, Mollusca, Echinodermata and Ascidiacea. Mature spicules appear to be extracellular structures. Sponge spicules initiate intercellularly then become extracellular. Alcyonarian, turbellarian, echinoid and ascidian spicule deposition begins intracellularly and then becomes extracellular. The continuation of growth in the extracellular environment has not been documented except for the echinoids. Placophoran spicules initiate and remain as extracellular structures. Early spicule growth seems to occur from or within a single cell. However, cell aggregation and/or neighboring cells appear to be important to the process of spicule formation. The spicule forming cells, in general, are found in a collagenous medium which may be associated with spicule growth. The organic matrix from the spicules of the gorgonian Leptogorgia virgulata is a glycoprotein. Autoradiography reveals that this matrix is apparently synthesized in the rough endoplasmic reticulum and Golgi complexes and then transported to the spicule forming vacuole via Golgi vesicles. To gain information about the entry and transport of calcium ions, the effects of ouabain and vanadate on calcium uptake were examined. Ouabain had no effect on calcium uptake. Vanadate treatment increased the uptake of calcium in scleroblasts and epithelial tissue and decreased its uptake in spicules. This may suggest that vanadate sensitive ATPases are involved in the pumping of calcium out of scleroblasts, out of epithelial cells into the mesoglea, and into scleroblast organelles. Autoradiography using 45Ca indicates that the majority of these ions initially accumulate in the branch axis. The label moves through the axial epithelium to the mesoglea and reaches the spiculeforming vacuoles in the scleroblasts via dense bodies