Capillary pressure can be viewed as the pressure required to drive a fluid through a pore throat and displace the pore wetting fluid, with greater pressure being required as the pore throat becomes smaller. The size and distribution of pore throats within a host rock control its capillary pressure characteristics, which in turn control fluid behavior in the pore system. Mercury-injection capillary pressure curves, which measure capillarity, are obtained by injecting mercury into sample plugs to produce a plot of injection pressure vs. mercury saturation. The resulting capillary pressure curves can provide qualitative assessments of reservoir rock using such calculated values as pore-throat sorting, reservoir grade, and oil columns required to obtain 50 and 75% oil saturat ons. Regional capillary pressure maps of these calculated values identify a north-south trend of favorable reservoir rock along the state line between Montana and North Dakota in the third porosity ("C") zone of the Ordovician Red River Formation. Oil migration and trapping can be significantly controlled by capillary pressure. At Bell Creek field, in southeastern Montana, multiple oil-water contacts in Cretaceous Muddy Sandstone barrier sands can be explained with the concept of a capillary pressure release valve. Semi-permeable siltstones can only trap finite columns with the excess oil being released updip to migrate into other reservoir quality sand bodies. Oil columns can be estimated from capillary pressure data by determining the buoyancy force of the oil floating on water and the converting from a mercury-air capillary system to an oil-water capillary system. Relative permeability data may also be calculated from capillary pressure data and can be used to estimate fractional water production at given water saturations. Tilted oil-water contacts can result from hydrodynamics, capillary pressure, or both, and capillary pressure data can help determine the type of tilting present. By understanding capillarity, we can obtain better insight on the nature of reservoir rock, oil accumulations, and permeability relationships.