Detecting pore fluids and their effects are the ultimate goals of almost all geophysical and petrophysical investigations. We strive to identify fluid types, extract their properties, predict pore pressures, and detect fluid motion. The important aspects of fluids are as much a function of their chemistry, distribution, and mobility as well as their simple bulk properties. Common pore fluids include hydrocarbons, carbon dioxide, drilling mud filtrates, water, and brines. Properties can be quite diverse. Hydrocarbons can range from light gases to semi-solid heavy oils. Heavy oils are of particular interest recently because of their new significance as a dominant hydrocarbon resource. Seismic properties of these fluids, such as bulk and shear moduli (yes, heavy oils have a shear modulus) are dominated by the content of high-molecular-weight components. As we broaden our search for new energy sources and as our geophysical data and resolution continue to improve, we will also need to be concerned with issues such as phase changes, fluid-rock interaction, compositional heterogeneity, and thermal effects.