This paper observes the following three challenge to biodiversity monitoring for REDD: choosing which aspects of biodiversity to monitor, the difficulty of attributing particular changes to REDD+ and the likely scarcity of resources for biodiversity monitoring. It proposes three responses to address these challenges: 1) agreed policy targets that identify what should be monitored; 2) making links to existing biodiversity monitoring, and to monitoring for estimates of GHG emissions and removals; and 3) developing clear theories of change to assist in determining which changes in biodiversity can be attributed to REDD+.