The painted decorations of Ostia, the harbour of Rome, are known by all for their excellent preservation state and for the importance they represent as one of the only testimonies of Ancient Wall Painting from the second to the fifth c. AD. However, it is surprising to see they have never been studied through a technical point of view, using them as a medium to understand the production dynamics of the workshops. This paper will try to reconstruct the history of Roman Painting in Ostia and to analyse its changes, not only through a stylistic perspective but also on an archaeological one. Recent studies have shown how the work habits of the painters and the materials change used through the centuries. In the Republican and Alto-imperial times, local workshops of very high level operated in various points of the city, painting decorations worthy of the richest domus in Rome. Between the first and the second c. AD, the status itself of wall painting changes in Ostia, where we see a radical change in the technique and the quality of the decorations. This paper will try to understand the reasons of that changing and to reconstruct the working of Ostian painters through time.