Data leaks create what is otherwise least associated with tax havens: transparency. Tax haven data leaks have occurred repeatedly and with increasing frequency since 2013, raising a natural question: What can we learn from data leaks about tax havens? We provide a practical guide for understanding and working with data leaks, highlight insights that have been gained from them, and illustrate the potential uses of the leaked data, while outlining avenues for future research. We focus on major tax haven data leaks over the 2013 to 2021 period, and particularly on the five large data leaks covered by the International Consortium of Investigative Journalists (ICIJ), which cover more than 800,000 offshore corporations, tens of millions of documents, and more than 8 terabytes of data: the Offshore Leaks, the Luxembourg Leaks, the Panama Papers, the Bahamas Leak, the Paradise Papers Leak and the Pandora Papers.