Mobile money schemes have grown rapidly in some developing countries but failed in many more. This paper reports the results of an empirical study of mobile money schemes in 22 developing countries chosen based on prior evidence to include roughly equal numbers of successes and failures. It uses a combination of quantitative and qualitative evidence to determine why some countries succeeded in launching mobile money schemes and others failed. The analysis is guided by multi-sided platform economics and in particular recent work on the role of ignition and critical mass. It finds among other things heavy regulation, and in particular an insistence that banks play a central role in the schemes, which is generally fatal to igniting mobile money schemes.