Realization, for U.S. Federal income tax purposes, is
a requirement in determining what must be included as income subject to taxation.
It should not be confused with the separate concept of Recognition (tax).

Defining Income
Realization is a trigger for calculating income taxation.
It is one of the three principles for defining income
under the seminal case in this area of tax law, Commissioner v. Glenshaw Glass Co.
In that case, the Supreme Court determined that income, for Federal income tax purposes,
generally means "undeniable accessions to wealth, clearly realized, and over which the taxpayers have complete dominion."
It is also discussed in Helvering v. Bruun, 309 U.S. 461 (1940), where the court explains that
“the realization of gain need not be in cash derived from the sale of an asset.
Gain may occur as a result of exchange of property, payment of the taxpayer's indebtedness,
relief from a liability, or other profit realized from the completion of a transaction.”
This is a checklist of types of realization triggers, but it is not an exhaustive list.
Finding something of value can also be a realization trigger, as the case of Cesarini v. United States demonstrates.

Problems in line-drawing
Realization is generally straightforward,
but there are instances at the margins in which the moment of realization can be tricky.
One example of a tricky realization situation which has given rise to substantial debate is
the 62nd home run ball hit by Mark McGwire.
The ball was retrieved by a grounds crewman, Tim Forneris.
Forneris gave McGwire the ball immediately after the game, amidst speculation that the ball could fetch
at least $1 million in an auction. Do either McGwire or Forneris have gross income?
Did Forneris realize income when he caught the ball? Tax professors typically teach
that it would be income to Forneris when he caught it, because it is Treasure trove.
As a result, the person who catches a home run ball would generally be required to include the value of the ball
in income in the year in which the catch took place, whether or not the person sold the ball,
and whether or not he gave it back to the player or the team.
This is an unpopular result, and the Internal Revenue Service (IRS) issued Ruling 98-56 to change the result
in the face of public pressure, but only in the case in which the player returned the ball.
Under this theory, an individual who catches a record-breaking ball has income at the very moment he possesses it,
unless he immediately disclaims possession by returning it.
If he does not do that, the only remaining question is what value he ought to include in income.
Because the treasure trove rule is that the value is that which exists at the time the ball is "reduced to possession,"
the answer must be a reasonable estimate of its market value, whether or not the recipient sells the ball.

アメリカ合衆国連邦所得税の目的において、
実現とは、課税される所得として何を含めなければならないかを決定するための必要条件である。
実現を別の概念である認識（税務）と混同してはならない。

所得を定義する
実現は、所得税額の計算事由である。
税法のこの分野において強い影響力を持っている事例であるコミッショナー対グレンショー・グラス社事件において、
実現は所得を定義するための3つの原則の1つである。
この事例において、最高裁判所は、アメリカ合衆国連邦所得税の目的を鑑みると、
所得とは通常、「富への否定できない増加物、はっきりと実現したもの、そして納税者が完全に支配権を持っているもの」
を意味する、と判決した。
所得に関しては、ヘルベリング対ブルーン事件（309 U.S. 461、1940年）においても議じられ、
その事件では、裁判所は以下のように説明を行った。
「収益の実現は、資産の売却から生じる現金である必要はない。
収益は、資産の交換、納税者の負債を他者が支払ったこと、債務の免除を受けたこと、もしくは取引の完了から実現する他の利益
の結果として生じるであろう。」
これは、実現事由種別のチェックリストとなっているが、全てを網羅したリストではない。
価値がある何かを認識することも実現事由となり得る、とセサリニ対アメリカ合衆国事件の事例では例証している。