The PAD-US Partnership has adopted the following definition:

Protected Areas are lands dedicated to the preservation of biological diversity and to other natural, recreation and cultural uses, and managed for these purposes through legal or other effective means. (adapted from IUCN definition)

In the PAD-US, protected areas are lands held in fee ownership for permanent or very long term open space uses. These include national parks and forests, public lands generally, wildlife preserves, state and local parks and reserves, lands held by non-profit organizations and many other areas. Eventually, the PAD-US Partnership will seek to incorporate marine protected areas, as well as lands that may only be secured for shorter time frames. The PAD-US does not generally include military or tribal lands unless they are secured for open space purposes.

Easements are also considered forms of protection and separate efforts are underway to develop better inventories of these areas, coordinated with the PAD-US.

Defining the level of "protection" for any particular area usually means assessing and applying USGS Gap scores (or IUCN rankings), which indicate the degree to which any land is managed to achieve biodiversity conservation.

Currently, there are two national inventory data sets being published:

The USGS Gap Analysis Program publishes PAD-US ...

The Conservation Biology Insitute (CBI) publishes PAD-US (CBI Edition) ...

In prior years, each organization has published related but now superseded data inventories:

• The Conservation Biology Institute's Protected Areas Database (CBI PAD) was for many years the only national level inventory of protected lands. First created in the late 1990s, the PAD was updated several times, as funding resources were available.
• The USGS Gap Analysis Program develops a wide range of conservation data and supports local conservation assessments (Gap analyses). For many years, its GAP Stewardship Layer was its primary means of inventorying protected areas. This data mainly contained federal and state lands, and was updated periodically by state, then by multi-state region. During the work of the PAD-US Partnership in 2008-2010, USGS published its first national inventory as PAD-US version 1 in April 2009.

There are six "gaps" in our current data inventory efforts, as identified in the 2009 PAD-US Partnership report, A Map for the Future:.

1. Funding - there is no assured funding to maintain a complete and current protected lands database of the U.S., and special investment is needed to make the jump from where we are now to the next level.
2. Completeness - we need to extend inventories to the local (city, county, metropolitan) levels, as well as to improve some state's data and some federal data.
3. Accuracy - the spatial accuracy of inventoried lands needs to be improved (possibly through better use of tax parcel data)
4. Standards and Process - standard collection models and processes need to be embraced at state and sub-state levels, to better facilitate a national dataset.
5. Conservation Status Coding - the protected lands inventory needs to more effectively support conservation status codes (e.g., Gap and IUCN scores) and needs an adjudication process to resolve issues in such codes.
6. Main Attributes - more complete and accurate descriptors about protected areas are needed (e.g., recreation access codes, management designations, etc.)

State Heritage Programs - each state has a heritage program office that helps develop and manage critical data for biodiversity planning and protection. Coordinated through NatureServe, many of these heritage programs play important roles in gathering protected lands data and will be part of any PAD-US strategy.

The Nature Conservancy Conservation Measures Program - TNC is investing considerable effort to improve the manner in which it assess conservation effectiveness of all types of land protection. TNC was actively involved with the PAD-US Partnership and remains a contributor to both ongoing CBI and USGS data efforts.

LandScope America - this collaboration between NatureServe and National Geographic Society showcases a wide range of conservation information and includes interactive maps. The Landscope project, however, is focused on integrating existing data rather than improving or developing new data.

Conservation Registry - Defenders of Wildlife has developed this web framework to better identify a range of on the ground conservation initiatives in the U.S., particularly those stemming from state wildlife action plans. The Registry may provide opportunities for feeding data into an improved national inventory process.