The Arch of Trajan in Leptis Magna, placed along the so-called Via Triumphalis to commemorate the grant of colonial status in 109-110 AD, is a concrete example of the different meanings of honorary architecture of North Africa, from the urbanistic one to the symbolic, political and propagandistic ones. Moreover, as a building belonging just to Lepcis, still entirely made of local stone, it has a central place in the architectural history of a city which is going to be characterized by a monumental growth and by the use of imported marble architectural elements since the late age of Hadrian: local building techniques, influences from Magna Graecia and Italy through the mediation of Sicily, Roman models (the arch of Nero in Rome seems to be the main typological model), Alexandrian and Cyrenaic elements are the outline conditions of a very original architecture. The aim of these notes, moving from a short history of studies, is to give a summary about the knowledge of the monument, trying to contextualize it: they analyze its features and illustrate the data come to light till now in a new research about it.