This chapter examines the basic components of natural mud suspensions and the main physical parameters used to characterize them. For more complex fluids such as mud suspensions it is also useful to try to relate their rheological properties as closely as possible to their microstructure and physical parameters. Clay particles originate in the physical or mechanical disintegration of rocks followed by a chemical transformation. For very dilute clay-water systems, particle interactions may be negligible. In that case it is worth noting that diffusion due to Brownian motion may tend to counteract sedimentation. A natural mud suspension consists mainly of water and grains whose size ranges from about 0.1 µm to about 10 m. The grain group includes various particle types consisting of inert mineral and rock fragments which may be roughly distinguished by their size.