This paper explores harmonisation of international regimes, in particular the factors that impact on it in practice. These issues are explored in the context of the harmonisation project undertaken by HELCOM in 2000–01. As will be demonstrated there are four main factors that influence the likelihood of harmonisation of measures. Of these the one most likely to promote harmonisation is pragmatism. The other three—geographic, economic and political factors—are all more likely to tilt against harmonisation than for it.