The Roman Empire had a Secret Service organized in two parts: one was military and the other was civilian. The two parts served the Emperor indirectly. The main troops engaged in the intelligence products were the frumentarii, the speculatores, the exploratores and the Praetorian Guard. The appointment of the frumentarii as part of the “Roman Secret Service” is mentioned during Hadrian’s time. They were perpetuated as far as Diocletian’s reign, who transformed the troops in the agentes in rebus. The present paper deals only with the “Intelligence Service” of the Principate.