Increasing efforts have recently focused on development of indicators for monitoring biodiversity loss, stimulated by development of the “2010 target.” Such efforts have failed to consider Goodhart's Law, which states that once an indicator is made into a policy target, then it will lose the information content that qualifies it to play its role as an indicator. The implications of Goodhart's Law for monitoring biodiversity are examined with specific reference to the IUCN Red List Index (RLI). According to Goodhart's Law, use of the RLI as an indicator could affect how conservation actions are targeted and how the Red List assessment is conducted, potentially undermining the assessment process itself. The use of targets in conservation policy and the associated development of indicators should therefore be undertaken with caution. Specifically, to support monitoring of global biodiversity loss, systems should be put in place to prevent the manipulation of indicators and the assessments on which they are based, to ensure that the information they provide is objective and reliable.