Although there have been finds to indicate that the site was inhabited in Neolithic times, Jerash rose to prominence from the time of Alexander the Great (333 BC).
In the wake of the Roman general Pompey's conquest of the region in 64 BC, Gerasa became part of the Roman province of Syria and then a city of the Decapolis. Over the next two centur-ies, trade with the Nabataeans (the creators of Petra) flourished and the city grew extremely wealthy. Local agriculture and iron-ore mining in the Ajlun area contributed to the city's wellbeing. A completely new city plan was drawn up in the 1st century AD, centred on the classical features of a colonnaded main north-south street intersected by two side streets running east-west.
When the emperor Trajan annexed the Nabataean kingdom (around AD 106) more wealth found its way to Gerasa. Many of the old buildings were torn down to be replaced by more imposing structures. Construction again flourished when Emperor Hadrian visited in AD 129. To mark a visit of such importance, the Triumphal Arch (now known as Hadrian's Arch) at the southern end of the city was constructed.
Gerasa reached its peak at the beginning of the 3rd century, when it was bestowed with the rank of Colony. Its ascendancy was, however, short lived - disturbances such as the destruction of Palmyra (Syria) in 273, the demise of the overland caravans and the development of sea trade pushed the city into a slow decline. The only respite came during the reign of Diocletian (around 300), which saw a minor building boom.
From the middle of the 5th century, Christianity was the major religion and the construction of churches proceeded apace. Under the Byzantine emperor Justinian (527-65) seven churches were built, mostly out of stones pillaged from the earlier Roman temples and shrines.
With the invasion of the Sassanians from Persia (now Iran) in 614, the Muslim conquest in 636 and the devastating earthquake in 747, Gerasa's glory days passed into shadow and its population shrank to about one-quarter of its former size.
Apart from a brief occupation by a Crusader garrison in the 12th century, the city was completely deserted until the arrival of the Circassians from Russia in 1878, after which the site's archaeological importance was realized and excavations began.