Collagens are the proteins found in the extracellular matrix of multicellular organisms, from primitive sponges (parazoans) to highly advanced mammals (metazoans). These proteins, classified as the “collagen superfamily," comprise about 28 members each with at least one triple-helical domain. Collagens deposited in the extracellular matrix (ECM) and connective tissues form supramolecular assemblies and function as structural proteins contributing to mechanical properties, organization, and shape of tissues. They regulate cell proliferation, migration, and differentiation by binding to cognate receptors on the cell surface and triggering signal transduction cascades. Collagens with restricted tissue distribution perform specific biological functions. This chapter envisages the structural and functional characteristics of both invertebrate and vertebrate collagens with a special account of sponge collagens and their significance in tissue engineering. © Springer India 2016.