Oceanic trenches are sites of convergence between two lithospheric plates, where one plate normally moves beneath the other to be eventually destroyed or incorporated into the mantle. These convergent margins are perhaps among the most striking and complex of the Earth’s tectonic features. This chapter examines the tectonics associated with the northern terminus of the trench, between 12°S and 18°S. The Tonga Trench is the northern portion of the linear Tonga-Kermadec Trench-Arc system, which is approximately 1400 km in length, 55 km in width, and has a maximum depth of 10,800 m. The various tectonic elements of the northern Tonga Trench region, with the exception of the islands of Samoa, are oriented nearly parallel to the trench and appear to have maintained the same general strike as they evolved. The first geophysical studies of the Tonga Trench were carried out in the early 1950s and 1960s before the acceptance of plate tectonics as a scientific paradigm.