Eminent domain seems like an awfully broad power--and indeed it is. Federal, state, and local governments can legally take private land from private people without their consent (though with some compensation).
The law says that such takings must be for “public use.” However, courts define “public use” so broadly that it is fairly easy for a government to establish that it intends to use the land in question for some societal benefit.
Some “public uses” are easy to understand: the government wants to construct a public park, fire station, or highway, which absolutely needs to incorporate your land. These uses would benefit the entire community, perhaps increasing health, safety, commerce, or public space.
But not all public uses need to benefit the entire public in order to allow legitimate uses of eminent domain, nor do they even need to be open to the entire public. For example, constructing a military base or government office would in all likelihood constitute a “public use,” even though most of the public would not be permitted to enter these facilities.
Even transfer of land to another private owner has been upheld by courts as a "public use," typically in situations where the purpose is to combat blight or promote economic development or urban revitalization.
In short, U.S. courts allow the government tremendous leeway in deciding what constitutes a “public use.” They primarily require that the government show some rational relationship between the intended use and the public benefit. As you might imagine, it is not usually difficult for the government to show this rational relationship, and therefore show a public use.
This naturally creates concern and criticism about use and abuse of the government's eminent domain power. Many allege that a situation has been created where private developers can influence the governmental process so as to reap benefits to someone else's detriment--building, for example, luxury condos in an area that once had middle-class single-family homes.
Go to the main eminent domain FAQ page.