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Goodwill in accounting is an intangible asset that arises when a buyer acquires an existing business. Goodwill represents assets that are not separately identifiable. Goodwill does not include identifiable assets that are capable of being separated or divided from the entity and sold, transferred, licensed, rented, or exchanged, either individually or together with a related contract, identifiable asset, or liability regardless of whether the entity intends to do so. Goodwill also does not include contractual or other legal rights regardless of whether those are transferable or separable from the entity or other rights and obligations. Examples of identifiable assets that are not goodwill include a company’s brand name, customer relationships, artistic intangible assets, and any patents or proprietary technology. The goodwill amounts to the excess of the "purchase consideration" (the money paid to purchase the asset or business) over the total value of the assets and liabilities. It is classified as an intangible asset on the balance sheet, since it can neither be seen nor touched. Under US GAAP and IFRS, goodwill is never amortized. Instead, management is responsible for valuing goodwill every year and to determine if an impairment is required. If the fair market value goes below historical cost (what goodwill was purchased for), an impairment must be recorded to bring it down to its fair market value. However, an increase in the fair market value would not be accounted for in the financial statements. Private companies in the United States, however, may elect to amortize goodwill over a period of ten years or less under an accounting alternative from the Private Company Council of the FASB.
In order to calculate goodwill, the fair market value of identifiable assets and liabilities of the company acquired is deducted from the purchase price. For instance, if company A acquired 100% of company B, but paid more than the net market value of company B, a goodwill occurs. In order to calculate goodwill, it is necessary to have a list of all of company B's assets and liabilities at fair market value.
Fair market value Accounts Receivable $10 Inventory $5 Accounts payable $6 Total Net assets = $10 + $5 - $6 = $9
In order to acquire company B, company A paid $20. Hence, goodwill would be $11 ($20 - $9). The journal entry in the books of company A to record the acquisition of company B would be:
DR Goodwill $11 DR Accounts Receivable $10 DR Inventory $5 CR Accounts Payable $6 CR Cash $20
Goodwill is a special type of intangible asset that represents that portion of the entire business value that cannot be attributed to other income producing business assets, tangible or intangible. 
For example, a privately held software company may have net assets (consisting primarily of miscellaneous equipment and/or property, and assuming no debt) valued at $1 million, but the company's overall value (including customers and intellectual capital) is valued at $10 million. Anybody buying that company would book $10 million in total assets acquired, comprising $1 million physical assets and $9 million in other intangible assets. And any consideration paid in excess of $10 million shall be considered as goodwill. In a private company, goodwill has no predetermined value prior to the acquisition; its magnitude depends on the two other variables by definition. A publicly traded company, by contrast, is subject to a constant process of market valuation, so goodwill will always be apparent.
While a business can invest to increase its reputation, by advertising or assuring that its products are of high quality, such expenses cannot be capitalized and added to goodwill, which is technically an intangible asset. Goodwill and intangible assets are usually listed as separate items on a company's balance sheet.
Previously, companies could structure many acquisition transactions to determine the choice between two accounting methods to record a business combination: purchase accounting or pooling-of-interests accounting. Pooling-of-interests method combined the book value of assets and liabilities of the two companies to create the new balance sheet of the combined companies. It therefore did not distinguish between who is buying whom. It also did not record the price the acquiring company had to pay for the acquisition. Since 2001, U.S. Generally Accepted Accounting Principles (FAS 141) no longer allows the pooling-of-interests method.
Goodwill is no longer amortized under U.S. GAAP (FAS 142). FAS 142 was issued in June 2001. Companies objected to the removal of the option to use pooling-of-interests, so amortization was removed by Financial Accounting Standards Board as a concession. As of 2005-01-01, it is also forbidden under International Financial Reporting Standards. Goodwill can now only be impaired under these GAAP standards.
Instead of deducting the value of goodwill annually over a period of maximal 40 years, companies are now required to determine the fair value of the reporting units, using present value of future cash flow, and compare it to their carrying value (book value of assets plus goodwill minus liabilities.) If the fair value is less than carrying value (impaired), the goodwill value needs to be reduced so the carrying value is equal to the fair value. The impairment loss is reported as a separate line item on the income statement, and new adjusted value of goodwill is reported in the balance sheet.
When the business is threatened with insolvency, investors will often deduct the goodwill from any calculation of residual equity because it will likely have no resale value.