Understanding the successes and failures of management of protected areas is vital for the conservation of global biodiversity. The Management Effectiveness Tracking Tool (METT) is a simple, questionnaire-based approach for assessing protected area management effectiveness (PAME). Since it was developed in 1999, it has become the most widely applied PAME tool, used in at least 127 countries worldwide. This paper reviews the development of the METT and how it has been implemented and adapted. A combination of literature review on implementation and implementation experience from the original authors and key users of the METT confirms that the METT is a relatively quick and simple way of collecting information about the status and trends of management in protected areas, and provides information to help drive management improvements. As such it is suitable for protected area managers, national protected area agencies, donors, and NGOs aiming to improve area management, and as a component of national reporting to the Convention on Biological Diversity. The paper examines issues related to METT implementation and concludes with 12 recommendations, from using the METT to verification of results, which together help ensure the tool is implemented in the most effective way and improves the credibility of PAME assessments.