In recent years, applications (apps) for iPads are increasingly being used to support learning in primary school contexts. Current estimates put the number of available educational apps at the iTunes store at approximately 500,000. Many of these apps contain mathematical content and purport to improve students' mathematics ability. Despite their availability, overall ease of use, and low price, significant questions remain as to their appropriateness in helping students develop mathematical knowledge. Three quantitative measures, previously used in other research to investigate digital technology use and student learning, were modified to meet the specific demands of evaluating apps. This chapter reports on the findings of a long-term research project that comprehensively reviewed mathematical apps to determine their usefulness for primary school students. It found that although the majority of apps provide little more than edutainment, a core group of apps were highly effective in supporting students in their development of mathematics knowledge.