Periodic review of an MEA’s effectiveness can help to ensure that the MEA is meeting or moving toward its desired goals. Such reviews may be mandated by provisions in the MEA. Alternatively, the Parties to an MEA may initiate such a review through mutual agreement, for example by decisions at the Conference of the Parties (COP) (e.g., see following case study on CBD). The review may be broad, reviewing all aspects of the MEA; or it could focus on specific aspects that Parties identify as entailing particular difficulties in compliance and enforcement.
Indicators provide one framework for assessing the effectiveness of an MEA. Performance indicators, on the one hand, can help to identify areas where Parties are 
complying with their obligations and areas where compliance is problematic. These indicators seek to answer questions related to: “Are countries doing what they said they would?” These indicators tend to assess governmental actions (development of laws, institutions, etc.) and sometimes private sector actions. Output indicators, on the other hand, seek to address whether the measures are actually effective. These indicators seek to answer questions related to: “Is the MEA having the desired effects?” These indicators often assess environmental conditions (as well as social and economic, depending on the MEA).
Usually, both types of indicators are necessary. Even if the performance indicators show that States are doing everything they said they would, this might not be enough and environmental conditions continue to degrade. In this instance, the MEA may need to be amended (for example, through a protocol) to take more stringent measures to protect the environment. Alternatively, States may be taking alternative measures to address the environmental problem. In which case, the performance indicators might indicate poor compliance, but the output indicators would suggest that the environmental conditions are doing well. In this situation, the MEA may need amendment to reflect alternative ways of complying so that States are not deemed to be in non-compliance when the goals are being met.