A recent English education policy has been to encourage state primary schools to become academies: state-funded, non-selective, and highly autonomous establishments. Primary schools have been able to opt-in to academy status since 2010 and academies now account for twenty-one percent of the primary sector. This paper investigates the causal effect of becoming a converter academy on primary school assessment outcomes, and on entry-year intake composition. Unlike existing evidence focused on earlier academies formed from failing secondary schools, no evidence is found of a converter academy effect on attainment for the average pupil. Although, there is evidence of a slight positive effect on age 11 attainment for pupils eligible for free school meals. There is no evidence that becoming a converter academy affects the composition of the entry-year intake.