Conversion is an intentional tort consisting of "taking with the intent of exercising over the chattel an ownership inconsistent with the real owner's right of possession". In England & Wales, it is a tort of strict liability. Its equivalents in criminal law include larceny or theft and criminal conversion. In those jurisdictions that recognize it, criminal conversion is a lesser crime than theft/larceny.

Examples of conversion include: 1) Alpha cuts down and hauls away trees on land s/he knows is owned by Beta, without permission or privilege to do so; and 2) Gamma takes furniture belonging to Delta and puts it into storage, without Delta's consent (and especially if Delta does not know where Gamma put it). A common act of conversion in medieval times involved bolts of cloth that were bailed for safekeeping, which the bailee or a third party took and made clothes for their own use or for sale.

Many questions concerning joint ownership in enterprises such as a partnership belong in equity, and do not rise to the level of a conversion. Traditionally, a conversion occurs when some chattel is lost, then found by another who appropriates it to his own use without legal authority to do so. It has also applied in cases where chattels were bailed for safekeeping, then misused or misappropriated by the bailee or a third party.

Conversion, as a purely civil wrong, is distinguishable from both theft and unjust enrichment. Theft is obviously an act inconsistent with another's rights, and theft will also be conversion. But not all conversions are thefts because conversion requires no element of dishonesty. Conversion is also different from unjust enrichment. If one claims an unjust enrichment, the person who has another's property may always raise a change of position defense, to say they have unwittingly used up the assets they were transferred. For conversion, there always must be an element of voluntarily dealing with another's property, inconsistently with their rights.