The Hellenic Trench (HT) is an oceanic trough located in the forearc of the Hellenic arc, an arcuate archipelago on the southern margin of the Aegean Sea plate, or Aegean Plate, also called Aegea, the basement of the Aegean Sea. The HT begins in the Ionian Sea near the mouth of the Gulf of Corinth and curves to the south, following the margin of the Aegean Sea. It passes close to the south shore of Crete and ends near the island of Rhodes just offshore Anatolia.

In the classical theory of its origin the HT is an oceanic trench containing the Hellenic subduction zone, directly related to the subduction of the African plate under the Eurasian plate. Alternate views developed later on additional data question the classical view postulating that the HT may be the result wholly or partially of back-arc extension and slab rollback. The "partial" view hypothesizes that the western leg of the HT, Ionian Sea east to eastern Crete, exhibits the line of subduction and therefore is an oceanic trench. The "not at all" view, relying on the theory that the subduction line is under or south of the Mediterranean Ridge, questions whether any of the HT is currently subductional. If not, it is merely a legacy, a remnant of a previous subduction zone that has gone elsewhere.

North of this subduction the Adriatic or Apulian Plate subducts under the Balkans. More recently and rarely the terms "North Hellenic Subduction" and "North Hellenic Trench" have been applied there, rendering the HT and HS into the "South HT" and "South HS." The distinction is based on a differentation of North Hellenides from South Hellenides. The dividing feature is the Gulfs of Patras and Corinth. From their vicinity and southward an extensional regime prevails, while the north remains in a compressional. The Hellenides are the mountains of Greece, divided into an inner and outer range. The extensional regime cuts across them transversely, producing four quarters. The South Hellenic Subduction Zone, and the Hellenic Trench, if different (many still consider them not to be so) are located in the southern outer Hellenides.

Meanwhile, the deep basins of the Trench and their marine ecologies are the homes of a number of marine mammals, such as Cetaceans, some of which are endangered species threatened by maritime traffic in the Eastern Mediterranean.

The study of the overall features of the surface of the Earth has been the concern of plate tectonics since the Plate Tectonics Revolution of the 1970s. It was a development of the continental drift theory of Alfred Wegener. These features are often called lineaments. The Hellenic Trench along with the Hellenic Arc and other related features are lineaments important to the geology primarily of Greece and secondarily of Turkey.

Morphology or geomorphology studies the "shapes" (morphai) of the lineaments, while kinesiology studies their "motions" (kineseis). Both topics as used typically in geology articles do not go beyond plane geometry, trigonometry, elementary algebra, and elementary statistics, which are taught at the high school level. More daunting are the geologic special terms, which are numerous, and continue to be innovated. This article assumes basic knowledge of mathematics and science, but includes parenthetical clues as to the meaning of the special terms as well as links to articles explaining them.

In subduction one plate dives under another at a convergent plate boundary and the band across this line is termed the subduction, or more rarely subductive, zone. It features an upper plate and a lower plate. The initial line of the subduction, traditionally believed to be located in the trench, and to be at the foot of the margin of the overriding plate, has a direction, the strike. The plate diving down does so at an angle, the dip. The direction of dip is roughly perpendicular, or normal, to the strike (not to be confused with a normal fault). It goes under the highlands raised by the collision, in this case the Hellenic arc. The two plates moving across each other (a dip-slip movement) generate earthquakes, so the subducted part of the plate is basically a seismic zone, called the Wadati–Benioff zone.

As it turns out, further research on the Hellenic Trench revealed that the concept of a subductive trench, where subduction is occurring now (if that is what it is), would only strictly apply to the west side; moreover, not all the subduction, wherever it does occur, is due to plate collision. The east side of the trench is not a trench but is a series of ascending scarps of faults where strike slippage is the main movement, due to further complexities discovered later (see this article, below).