Accession (from Lat. accedere, to go to, to approach), in law, a method of acquiring property adopted from Roman law, by which, in things that have a close connection with or dependence on one another, the property of the principal draws after it the property of the accessory, according to the principle, accessio cedet principali. Accession may take place either in a natural way, such as the growth of fruit or the pregnancy of animals, or in an artificial way. The various methods may be classified as (i) land to land by accretion or alluvion; (2) moveables to land (fixtures ); (3) moveables to moveables; (4) moveables added to by the art or industry of man; this may be by specification, as when wine is made out of grapes, or by confusion, or commixture, which is the mixing together of liquids or solids, respectively. In the case of industrial accession ownership is determined according as the natural or manufactured substance is of the more importance, and, in general, compensation is payable to the person who has been dispossessed of his property.

In a historical or constitutional sense, the term accession" is applied to the coming to the throne of a dynasty or line of sovereigns or of a single sovereign.

"Accession" sometimes likewise signifies consent or acquiescence. Thus, in the bankruptcy law of Scotland, where there is a settlement by a trust-deed, it is accepted on the part of each creditor by a "..deed of accession."