Over the last two decades the global number of transboundary protected areas complexes (TBPA) has gradually increased from 59 transborder areas (combined from 70 protected areas) as identified by IUCN in late 1980-es, to 169 (involving 666 individual protected areas) in 2001, and to 188 internationally adjoining protected areas (with at least 818 individual sites) by 2005.

The latest (April 2007) global inventory of transboundary protected areas and other sites, not necessarily ajusent but linked by various transboundary conservation initiatives has been implemented by the UNEP World Conservation Monitoring Centre (UNEP-WCMC). Inventory was based on reviewing the content (digital maps) of the World Database on Protected Areas (WDPA) and identified 227 TBPA complexes incorporating 3,043 individual protected areas or internationally designated sites.

The 2007 TBPA List incorporated both TBPAs that fit entirely into IUCN definition (confirmed international cooperative or sympathetic management through legal or other effective means) and internationally adjoining protected area (IAPA, following the Mittermeier et all., 2005) that may still require development of collaborative efforts or further formalizing of ongoing cooperation.

Considerable increase in the number of PA included into TBPA complexes was related both to much improved and expanded content of the WDPA, to accounting of overlapping sites, e.g. national and international or small in size protected sites (e.g. nature monuments often located within a larger PA) and other assumptions of the methodology applied.