Since 1997 Tanzania has undertaken a process to identify and declare a network of Nature Forest Reserves (NFRs) with high biodiversity values, mainly from within its existing portfolio of national Forest Reserves, but with some new extensions. In recent years this expansion has accelerated, with ten NFRs declared since 2015. The current network of 19 existing NFRs covered 918,212 hectares by the end of 2020 (with an additional three reserves covering 34,862 hectares in the process of being declared). The coverage by NFRs of Tanzanian habitat types has increased to include the main forest types, wet, seasonal, and dry, and includes wetlands and grasslands. This has led to more than a doubling of the coverage of species ranges of vertebrates by the NFR network. Declared and proposed reserves now contain at least 178 of Tanzania’s 242 endemic vertebrate species, of which over 50% are threatened with extinction, and 553 Tanzanian endemic plant taxa (species, subspecies, and varieties), of which 61.3% are threatened. NFRs also support 41 single site endemic vertebrate species and 76 single site endemic plant taxa. Time series data show that NFR management effectiveness is increasing, especially where donor funds are available. Tourism, diversified revenue generation and investment schemes are required to create a network of economically self-sustaining NFRs able to conserve critical biodiversity values. Improved management and investment have reduced some threats in recent years, but ongoing challenges include illegal logging, charcoal production, firewood, pole cutting, hunting, fire, wildlife trade, and the unpredictable impacts of climate change.