International debates on taxation and development have been informed by a popular narrative that there is a large ‘pot of gold’ for funding which could be released by cracking down on the questionable tax practices of multinational enterprises, and which could bridge the gap towards funding the sustainable development goals. How much of this is wishful thinking and how much really reflects what we know? This paper looks at the 'big numbers' that have shaped this debate and seeks to clarify the emerging evidence.