TX 75083-3836 U.S.A., fax 01-972-952-9435.Abstract. Primary and secondary migrations are well defined terms in hydrocarbon reservoir genesis. Both of them designate spontaneous displacement of the hydrocarbon, from the source rock to the final trap, i.e. to the reservoir rock.Producing the reservoir implies a forced movement of hydrocarbons towards the wells, consuming the energy accumulated over the ages.Mainly due to the gravitational forces and water influx, a new kind of fluid movement is taking place into the reservoir towards its production final stage. The main feature of the fluids displacements into the reservoir is their spontaneous character. This kind of fluid movement that occur after reservoir abandonment, defines the tertiary migration process.As a consequence of tertiary migration, the reservoir fluids, water, oil and gas, tend to place themselves to a normal-gravitational position. Additionally, the dynamic pressure distribution at the end of the production history tends to relax, amplifying the migration process towards normal stationary pressure gradients. The result of the tertiary migration is a new saturation distribution into the reservoir and it is expected that in the most favorable cases, the production potential of some parts of the reservoir should be restored.From this point of view, an enormous interest for Romania concerns the old oil reservoirs, abandoned over 70-80 years ago with a final recovery factor sometimes less then 20%.The present paper examines the principles and factors involved in the tertiary migration process. A special attention is paid to pressure redistribution and gravitational segregation during such a process. To demonstrate the concept, this study discusses two successful production restarting projects in Romania. If today the world is not very proud about production restarting projects, tomorrow will be for sure.