MARXAN is a family of software designed to aid systematic reserve design on conservation planning. With the use of stochastic optimisation routines (Simulated Annealing) Marxan generates spatial reserve systems that achieve particular biodiversity representation goals with reasonable optimality. Over the years, Marxan has grown from its standard two zone application to consider more complex challenges like incorporating connectivity, probabilities and multiple zones. Along the way, Marxan’s user community has also built plug-ins and interfaces to assist with planning projects.

Computationally, Marxan provides solutions to a conservation version of the 0-1 knapsack problem, where the objects of interest are potential reserve sites with given biological attributes. The simulated annealing algorithm attempts to minimise the total cost of the reserve system, while achieving a set of conservation goals (typically that a certain percentage of each geographical/biological feature is represented by the reserve system).

Marxan is a portmanteau acronym, fusing MARine, and SPEXAN, itself an acronym for SPatially EXplicit ANnealing. It was a product of Ian R. Ball's PhD thesis, while he was a student at the University of Adelaide in 2000, and was supervised and funded by Professor Hugh Possingham, the state of Queensland's (Australia) current Chief Scientist who holds a Federation Fellowship at the University of Queensland. It was an extension of the existing SPEXAN program.

In 2018, the vision of “Democratizing Marxan” began. Through the Biodiversity and Protected Areas Management programme (BIOPAMA), funded by the European Union, the Joint Research Centre worked closely with The Nature Conservancy to prototype a web-based Marxan platform that improves accessibility to non-experts and supports our common vision of providing accessible tools for evidence-based conservation planning. This led to a partnership with Microsoft in 2020, which aims to scale Marxan’s infrastructure for global accessibility and empowering users with the tools and data they need to make smarter decisions for the planet. In late 2020 and early 2021 Microsoft's Azure Quantum team made several open source contributions to Marxan resulting in increased performance when running on multi-core machines and cloud environments. The resulting version 4 of Marxan is now available from marxansolutions.org.

MARXAN is the most widely used systematic reserve planning software in the world,^[2] and has been used to create the marine reserve network on the Great Barrier Reef, in Queensland, Australia, the largest marine protected area in the world.^[3] It has been used for many other marine and terrestrial reserve planning applications.^[4]

• Channel Islands of California^[5]
• Gulf of Mexico^[6]
• Galapagos Islands
• South Australia^[7]
• British Columbia^[8]
• Connecticut/New York
• Central Coast of California
• Baltic Sea^[9]

Beyond protected area network design, MARXAN has been applied to hundreds of conservation planning challenges, from designing optimal poaching patrols for game reserves and identifying where to conserve essential ecosystem services, to helping with transboundary ocean planning and understanding where transnational collaborations might best be prioritized to achieve conservation goals. While it would be almost impossible to list all of MARXAN's applications, here are a few examples beyond protected area network design. For software specific examples, see the Software section.

• Restoration activities in the Atlantic Forest, Brazil,^[10] in the Yucatan Peninsula in the Mexican Caribbean,^[11] in the Murray–Darling Basin in South Australia,^[12] and southwestern Alberta, Canada^[13]
• Provision of ecosystem services in Central Coast ecoregion of California, United States,^[14] Telemark in southern Norway,^[15] and Vermont, United States^[16]
• Understanding trade-offs between competing objectives in the Andes of Bolivia,^[17] and Central Kalimantan, Indonesia^[18]
• Identifying management priorities in the Danube River Basin, Europe,^[19] and South Africa's grassland biome^[20]
• Law enforcement activities in the Greater Virunga Landscape, in central Africa,^[21] and the Patos Lagoon estuary along the Brazilian coast^[22]

MARXAN has been used extensively by The Nature Conservancy, and is a major part of the systematic planning tools being used in the Global Marine Initiative. The World Wildlife Fund used MARXAN to define a Global set of Marine Protected Areas, the Roadmap to Recovery, which they used to petition the UN about the creation of open ocean marine reserve networks.

The software has also been used in terrestrial applications, such as:

• The North American Wildlands Project.
• Selecting priority areas for Global Mammal Assemblages.^[23]
• Planning the conservation of ecosystem services.^[14]
• The Great Sand Hills of Saskatchewan Regional Environmental Study ^[24]