Spicules are structural elements found in most sponges. The meshing of many spicules serves as the sponge's skeleton and thus it provides structural support and potentially defense against predators.

Sponge spicules are made of calcium carbonate or silica. Large spicules visible to the naked eye are referred to as megascleres or macroscleres, while smaller, microscopic ones are termed microscleres. The composition, size, and shape of spicules are major characters in sponge systematics and taxonomy.

Sponges are a species-rich clade of the earliest-diverging (most basal) animals. They are distributed globally, with diverse ecologies and functions, and a record spanning at least the entire Phanerozoic.

Most sponges produce skeletons formed by spicules, structural elements that develop in a wide variety of sizes and three dimensional shapes. Among the four sub-clades of Porifera, three (Demospongiae, Hexactinellida, and Homoscleromorpha) produce skeletons of amorphous silica  and one (Calcarea) of magnesium-calcite. It is these skeletons that are composed of the elements called spicules. The morphologies of spicules are often unique to clade- or even species-level taxa, and this makes them useful in taxonomic assignments.

In 1833, Robert Edmond Grant grouped sponges into a phylum he called Porifera (from the Latin porus meaning "pore" and -fer meaning "bearing"). He described sponges as the simplest of multicellular animals, sessile, marine invertebrates built from soft, spongy (amorphously shaped) material.

Later, the Challenger expedition (1873–1876) discovered deep in the ocean a rich collection of glass sponges (class Hexactinellida), which radically changed this view. These glass sponges were described by Franz Schulze (1840–1921), and came to be regarded as strongly individualised radially symmetric entities representing the phylogenetically oldest class of siliceous sponges. They are eye-catching because of their distinct body plan (see lead image above) which relies on a filigree skeleton constructed using an array of morphologically determined spicules.

Then, during the German Deep Sea Expedition "Valdivia" (1898-1899), Schulze described the largest known siliceous hexactinellid sponge, up to three metres high Monorhaphis chuni. This sponge develops the also largest known bio-silicate structures, giant basal spicules, three metres high and one centimetre thick. With such spicules as a model, basic knowledge on the morphology, formation, and development of the skeletal elements could be elaborated. Spicules are formed by a proteinaceous scaffold which mediates the formation of siliceous lamellae in which the proteins are encased. Up to eight hundred 5 to 10 μm thick lamellae can be concentrically arranged around an axial canal. The silica matrix is composed of almost pure silicon and oxygen, providing it with unusual optophysical properties superior to man-made waveguides.

Since their discovery, hexactinellids were appraised as "the most characteristic inhabitants of the great depths", rivalling in beauty the other class of siliceous Porifera, the demosponges. Their thin network of living tissues is supported by a characteristic skeleton, a delicate scaffold of siliceous spicules, some of which may be fused together by secondary silica deposition to form a rigid framework. The Hexactinellida together with the Demospongiae forms a common taxonomic unit comprising the siliceous sponges. The spicules, the elements from which their skeletons are constructed, are built in a variety of distinct shapes, and are made from silica that is deposited in the form of amorphous opal (SiO₂·nH₂O).