Biological systems are well known for their ability to construct remarkably complex and mechanically robust skeletal structures from a great diversity of minerals. One such example, silica, is widely used in the synthesis of skeletal elements (spicules) within the phylum Porifera (the sponges). As a result, members of this diverse group have served as useful model systems for analysis of the dynamic processes of biosilicification and for investigating structure function relationships in their often hierarchically ordered skeletal systems. This article describes in detail the skeletal diversity within the two silica-forming sponge classes, the Demospongiae and the Hexactinellida, and through the use of several representative examples, discusses the mechanical consequences of the various modes of construction implemented as well as the potential evolutionary pressures that resulted in their observed structural complexity.