The issue that this article introduces grew out of an event, the UNESCO Mobile Learning Week, but also out of a wider and growing movement of people and organisations exploiting mobile technologies, as they pursue varied educational missions. The UNESCO Mobile Learning Week represented by contributions here was a focus for contributions from across the field. This article provides a wider discussion of these contributions, first by looking at the achievements of UNESCO and then by considering these achievements more critically. In particular, it highlights several sets of inherent challenges facing UNESCO and other organisations engaging in mobile learning: those around evidence, evaluation, and sustainability; the problematic tension between large-scale interventions, based on scale and content delivered by national governments, and the language and culture of marginal or indigenous people; the absence of appropriate ethics procedures to manage educational interventions delivered by a powerful and ubiquitous technology; and the absence of learners in the forums that address all these various issues.