[[Recovery]] of petroleum oil from underground [[reservoir|reservoirs]] by using secondary sources of energy, such as injected natural gas or water to increase producing rates and to improve ultimate oil recovery. Water injection, commonly known as [[water flooding]], usually affords higher recovery than gas injection. Gas injection is generally limited to those reservoirs which have a gas cap and in which gas cap expansion is expected to be an efficient natural drive mechanism.

Although the terms "primary" and "secondary" imply a sequence of use, both mechanisms might work concurrently once secondary recovery is implemented. See also [[primary recovery]] and [[tertiary recovery]].