This paper attempts a synthesis of theoretical and empirical work on international financial contagion. Although a professional consensus on the appropriate definitions of contagion has yet to emerge, we document substantial research progress towards this goal. On the empirical front, determining when returns are ‘excessive’ is a pre-condition for designing effective policy response to crises. At the theoretical level, tracing the observed herding behavior to market participants’ uncertain beliefs and information asymmetries is a key element for understanding how contagious effects arise. It is argued that the recent focus on better understanding of high-frequency financial returns data and decision making at the market microstructure level are promising avenues for understanding the transmission of shocks across markets and countries.