Many new exporters give up exporting very shortly, despite substantial entry costs; others shoot up foreign sales and expand to new destinations. We develop a model based on experimentation to rationalize these and other dynamic patterns of exporting firms. We posit that individual export profitability, while initially uncertain, is positively correlated over time and across destinations. This leads to "sequential exporting," where the possibility of profitable expansion at the intensive and extensive margins makes initial entry costs worthwhile despite high failure rates. Firm-level evidence from Argentina's customs, which would be difficult to reconcile with existing models, strongly supports this mechanism. Sequential exporting also has important and novel policy implications: a reduction in trade barriers has delayed effects, while also promoting entry in third markets. This trade externality poses challenges for the quantification of the effects of trade liberalization programs and implies that the consequences of international trade agreements are significantly richer than traditional models suggest. JEL Codes: F10; D21; F13 Keywords: Export dynamics, experimentation, uncertainty, learning, trade liberalization * We thank Costas Arkolakis, David Atkin, Sami Berlinski, Jordi Blanes-i-Vidal, John Bluedorn, Holger Breinlich, Svetlana Demidova, Nic de Roos, Peter Egger, Robert Elliott, Daniel Ferreira, Rodrigo Fuentes, Martin Gervais, Juan Carlos Hallak, James Harrigan, Beata Javorcik, Marc-Andreas Muendler, Peter Neary, Brent Neiman, Dimitra Petropoulou, Horst Raff, Steve Redding, Frédéric Robert-Nicoud, Mark Roberts, Ina Simonovska, Thierry Verdier, Zhihong Yu, and seminar participants at various institutions and conferences for valuable comments and suggestions. We also thank the support of the Chair Jacquemin of the Université Catholique de Louvain in choosing this paper for its annual award at the 2009 European Trade Study Group Meeting. We gratefully acknowledge financial support from the British Academy and the ESRC.