The integration of reservoir processes with horizontal well technology has led to the development of 'short-distance displacement' processes for more efficiently recovering heavy oil and tar sands bitumen. THAI, 'toe-to-heel' air injection, using horizontal producer wells in a line drive configuration, creates unique operating conditions in the reservoir that have special advantages for heavy oil recovery. Similar to the SAGD (steam assisted gravity drainage) and VAPEX (vapour extraction) processes, the distance that the mobilized oil has to travel to the producer well(s) in THAI is short (typically a few metres), compared with the long-distance (hundreds of metres) in a conventional flood pattern. SAGD, VAPEX and THAI are, therefore, classified as 'short-distance displacement' processes. In THAI, a gravity-assist mechanism exists by virtue of the mobile oil zone that develops immediately ahead of the combustion front. This occurs in a heavy oil reservoir when the vaporized oil and gases are prevented from communicating through the downstream oil zone and, instead, are forced to flow down into the exposed part of the horizontal producer well in line drive, i.e. short-distance displacement. The gravity-assist prevents gas override, ensuring an upright combustion front and hence a stable process. Three-dimensional combustion cell tests using heavy Wolf Lake (Alberta) oil have revealed the essential characteristics of THAI. The short-distance displacement provides for constant, stable process conditions in THAI. The thermal sweep efficiency of the process is very high and virtually all of the oil in the zones contacted by the combustion front is produced, except for the small fraction consumed as fuel. The potential oil recovery is very high compared with other processes. It also produces thermally upgraded oil, with an increase of 5-7 API points.