Effective measurement will be a key precursor to managing and meeting the post-2015 development goals. As the international community meets to thrash out the framework for monitoring these goals, we must take stock of lessons learnt from the Millennium Development Goal process. This paper highlights one lesson: the persistent differences between the MDG indicator estimates from national governments and those from international agencies. It investigates the extent, nature, and implications, of these differences. Crucially, it considers the reasons behind these differences and what this means for attempts to monitor progress towards the Sustainable Development Goals (SDGs).