We used aerial photographs, satellite images, and field surveys to monitor forest cover in the core zones of the Monarch Butterfly Biosphere Reserve in Mexico from 2001 to 2012. We used our data to assess the effectiveness of conservation actions that involve local, state, and federal authorities and community members (eg, local landowners and private and civil organizations) in one of the world's most iconic protected areas. From 2001 through 2012, 1254 ha were deforested (i.e., cleared areas had <10% canopy cover), 925 ha were degraded (i.e., areas for which canopy forest decreased), and 122 ha were affected by climatic conditions. Of the total 2179 ha of affected area, 2057 ha were affected by illegal logging: 1503 ha by large-scale logging and 554 ha by small-scale logging. Mexican authorities effectively enforced efforts to protect the monarch reserve, particularly from 2007 to 2012. Those efforts, together with the decade-long financial support from Mexican and international philanthropists and businesses to create local alternative income and employment, resulted in the decrease of large-scale illegal logging from 731 ha affected in 2005-2007 to none affected in 2012, although small-scale logging is of growing concern. However, regional social and economic problems remain, and they must be addressed to ensure the reserve's long-term conservation. The monarch butterfly (Danaus plexippus) overwintering colonies in Mexico-which engage in one of the long-known insect migrations-are threatened by deforestation, and a multistakeholder, regional, sustainable-development strategy is needed to protect the reserve.