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A megaproject is an extremely large-scale investment project. Megaprojects are typically defined as costing more than US$1 billion and attracting a lot of public attention because of substantial impacts on communities, environment, and budgets. Megaprojects can also be defined as "initiatives that are physical, very expensive, and public". Care in the project development process is required to reduce any possible optimism bias and strategic misrepresentation, as a curious paradox exists in which more and more megaprojects are being proposed despite their consistently poor performance against initial budget and schedule forecasts.
The logic on which many of the typical megaprojects are built is collective benefits; for example electricity for everybody (who can pay), road access (for those that have cars), etc. Megaprojects have undergone a wide criticism for their top down planning processes and for their ill effects on certain communities. From the 1960s on, mass mobilization took place against the building of inner city freeways in North America (for example in New York City, Toronto, Seattle, San Francisco), or nuclear power plants in the United States and Germany, or proposal for new airports such as Mexico City in 2001. More recently, new types of megaprojects have been identified that no longer follow the old models of being singular and monolithic in their purposes, but have become quite flexible and diverse, such as waterfront redevelopment schemes that seem to offer something to everybody. However, just like the old megaprojects, the new ones also foreclose "upon a wide variety of social practices, reproducing rather than resolving urban inequality and disenfranchisement". Because of their plethora of land uses "these mega-projects inhibit the growth of oppositional and contestational practices". The collective benefits that are often the underlying logic of a mega-project, are here reduced to an individualized form of public benefit.
Investing in megaprojects in order to stimulate the general economy has been a popular policy measure since the economic crisis of the 1930s. Recent examples are the 2008–09 Chinese economic stimulus program, the 2008 European Union stimulus plan, and the American Recovery and Reinvestment Act of 2009. Controversy often surrounds megaprojects, often in relation to their cost overruns. Bent Flyvbjerg has noted the existence of incentives to overstate income, underestimate costs, and exaggerate future social and economic benefits due to lack of accountability and risk-sharing mechanisms. 
One of the most challenging aspects of megaprojects is obtaining sufficient funding. Alan Altshuler and David Luberoff have found that creative and politically adept political leadership is required to secure resources as well as generate public support, mollify critics, and manage conflict through many years of planning, authorization and implementation. Other challenges faced by those planning megaprojects include laws and regulations that empower community groups, contested information and methodologies, high levels of uncertainty, avoiding impacts on neighborhoods and the environment, and attempting to solve a wicked problem.
Megaprojects include bridges, tunnels, highways, railways, airports, seaports, power plants, dams, wastewater projects, Special Economic Zones, oil and natural gas extraction projects, public buildings, information technology systems, aerospace projects, weapons systems, large-scale sporting events and, more recently, mixed use waterfront redevelopments; however, the most common megaprojects are in the categories of hydroelectric facilities, nuclear power plants and large public transportation projects.
- Reference class forecasting
- Optimism bias
- Megaprojects and Risk
- When Technology Fails
- World's Toughest Fixes TV series
- Alan Altshuler and David Luberoff, Mega-Projects: The Changing Politics of Urban Public Investment (Washington, DC: Brookings Institution, 2003). ISBN 0-8157-0129-2
- Bent Flyvbjerg, Nils Bruzelius and Werner Rothengatter, Mega-Projects and Risk: An Anatomy of Ambition (Cambridge UK, Cambridge University Press, 2003). ISBN 0-521-00946-4
- Lehrer, U.; Laidley, J. (2008). "Old Mega-projects Newly Packaged? Waterfront Redevelopment in Toronto". International Journal for Urban and Regional Research 32 (4): 786–803.
- Flyvberg, B., Bruzelius, N., Rothengatter, W. Megaprojects and Risk: An Anatomy of Ambition. Cambridge: Cambridge University Press
- Altshuler, Alan and David Luberoff. Mega-Projects: The Changing Politics of Urban Public Investment. Washington, D.C.: Brookings Institution, 2003.
- Plotch, Philip Mark. What’s Taking So Long? Identifying the Underlying Causes of Delays in Planning Transportation Megaprojects in the United States. Journal of Planning Literature. Available online January 8, 2015.