Just compensation is a right enshrined in the Fifth Amendment to the U.S. Constitution (and counterpart state constitutions), which is invoked whenever private property is taken by the government. Under some state constitutions, it is also owed when the government "damages" private property.
Usually, the government files an eminent domain action to take private property for public use and just compensation is determined at trial if the landowner does not settle with the government. However, when the government fails to file an eminent domain action and pay for the taking, the owner may seek compensation in an action called inverse condemnation.
For reasons of expedience, courts generally use fair market value as the measure of just compensation. The primary evidence of fair market value in almost all eminent domain are appraisal reports created by the appraisers hired by both sides. Because of the difficulty in stopping the eminent domain process, and the sole issue typically being what just compensation is owed to a landowner, an eminent domain trial is often called a "battle of the appraisers."