This chapter is devoted to materials in the fluid or liquid states and itis focused on the study of their physical properties at pressures below 1 GPa. This pressure level still marks the limit for most practical applications requiring an accurate knowledge of fluid properties and large-scale industrial processes. These areas of study span from simple liquids to very complex fluid systems related to living organisms and microbiology (Chapter 11), food technology (Chapter 12), cosmetic and pharmaceutical preparations (Chapter 13), geophysics and geochemistry (Chapter 15), and astrobiology (Chapter 16).