An estate is an interest in real property; it is the degree to which a thing is owned. Freehold estates are those with an uncertain duration; leaseholds are estates due to expire at a definite time. A present estate is one that is currently owned; a future estate is one that is owned now but not yet available for use.

Present estates are (1) the fee simple absolute; (2) the fee simple defeasible, which itself may be divided into three types, and (3) the life estate.

Future estates are generally of two types: reversion and remainder. A reversion arises whenever a transferred estate will endure for a shorter time than that originally owned by the transferor. A remainder interest arises when the transferor gives the reversion interest to someone else.

Use of air, earth, and water are the major rights incident to ownership of real property. Traditionally, the owner held “up to the sky” and “down to the depths,” but these rules have been modified to balance competing rights in a modern economy. The law governing water rights varies with the states; in general, the eastern states with more plentiful water have adopted either the natural flow doctrine or the reasonable use doctrine of riparian rights, giving those who live along a waterway certain rights to use the water. By contrast, western states have tended to apply the prior appropriation doctrine, which holds that first in time is first in right, even if those downstream are disadvantaged.

An easement is an interest in land—created by express agreement, prior use, or necessity—that permits one person to make use of another’s estate. An affirmative easement gives one person the right to use another’s land; a negative easement prevents the owner from using his land in a way that will affect another person’s land. In understanding easement law, the important distinctions are between easements appurtenant and in gross, and between dominant and servient owners.

The law not only defines the nature of the property interest but also regulates land use. Tort law regulates land use by imposing liability for (1) activities that affect those off the land and (2) injuries caused to people who enter it. The two most important theories relating to the former are nuisance and trespass. With respect to the latter, the common law confusingly distinguishes among trespassers, licensees, and invitees. Some states are moving away from the perplexing and rigid rules of the past and simply require owners to maintain their property in a reasonably safe condition.

Land use may also be regulated by private agreement through the restrictive covenant, an agreement that “runs with the land” and that will be binding on any subsequent owner. Land use is also regulated by the government’s power under eminent domain to take private land for public purposes (upon payment of just compensation), through zoning laws, and through recently enacted environmental statutes, including the National Environmental Policy Act and laws governing air, water, treatment of hazardous wastes, and chemicals.