The global spread of national assessment testing activities, and the growing pressure to move beyond basic measures of participation in educational monitoring, means that student achievement measures are likely to become increasingly relevant indicators of systemic progress in the developing world. Using data from the CESSP project in Cambodia, this paper incorporates a standard decomposition framework to go beyond simple comparisons of average achievement levels over time in order to better understand the underlying dynamics of change. The results show that recent improvements in student achievement in Cambodia are attributable in part to changes in the composition of student cohorts, although there is some evidence of a tradeoff between increasing participation rates and average achievement. There is also some encouraging evidence that school quality is improving, especially in lower grades where the leveling off of participation is creating a policy window of opportunity. The framework can be easily applied to a growing body of assessment data in the developing world to aid both inter- and intra-national monitoring of education system progress.