He is most famous today for two contributions to European history. The first concerns the Dark Ages, which had customarily been thought of as beginning with the decline of Roman imperial power in the West during the course of the fifth century. In his book Mohammed and Charlemagne, he pointed out the essential continuity of the economy of the Roman Mediterranean even after the barbarian invasions. This continuity, and the control of the internal trade routes on which imperial prosperity depended, was only cut with the Arab expansion in the seventh century, which forced the remainder of the Christian West into a kind of non-trading economy, giving rise to feudalism, the so-called "economy without outlets".
Pirenne attempted to use quantative methods in relation to currency in support of the thesis, which are easy to argue over in detail, and the thesis in general remains controversial. But it has shown the arbitrary nature of the periodisation of the beginning of the Middle Ages at the "fall" of the Western Roman empire.
His other major idea concerned the nature of medieval Belgium. Belgium as an independent nation state had been brought into being only a generation before Pirenne's birth; throughout Western history, its fortunes had been tied up with the Low Countries, which now include the Netherlands, Luxembourg and parts of north-east France. Furthermore, Belgium lies athwart the great linguistic divide between French and Dutch. The unity of the country could be thought accidental, something which Pirenne sought to disprove in his History of Belgium (1899-1932). His ideas here have also proved controversial, with many historians preferring to stress the economic unity of the Low Countries as a whole.