This chapter discusses the formation of inorganic particles and on the origin and processing of particulate organic matter. A In the open ocean, two major types of inorganic particles are formed: CaCO3 and opaline silica, SiO2. The bulk of inorganic particles in the open oceans, however, is composed of CaCO3 or SiO2 and result from biological processes. An additional means by which inorganic particles can be formed in aquatic ecosystems is through hydrous aluminum oxide formation when acidic aluminum-bearing waters are neutralized. The formation of inorganic particles within aquatic ecosystems results from the biological formation of hard body parts and exceeding the chemical solubility of specific compounds due to changes in pH, redox potential, or ionic concentrations Both pathways can be found to be active in certain zones of most aquatic ecosystems. Sources of organic particles may be difficult to determine because particles are usually substantially altered in size, shape, appearance, and chemical composition from the original source.