The Council of Australian Governments (COAG) has endorsed a "Closing the Gap" agenda designed to improve the lives of Indigenous Australians. The agenda is distilled in a series of targets, one of which is the goal of halving the gap for Indigenous students in Year 12 attainment or equivalent (Certificate II or above) attainment rates by 2020. One of the proposed measures for monitoring progress towards achieving this target is apparent secondary school retention. This paper reviews estimates of recent changes in the gap in apparent Year 12 retention between Indigenous and non-Indigenous students using data from the MCEECDYA National Schools Statistics Collection. It also provides some alternative estimates of Year 12 participation using ABS experimental estimates of the Indigenous population. It uses these estimates to explore the increases in Indigenous Year 12 retention required to halve the gap between Indigenous and non-Indigenous students and places these in the context of recent changes in the Year 12 retention of Indigenous students. A table is appended presenting an estimated resident Indigenous population. (Contains 4 tables.)