Almost every business must invest in some major equipment, vehicles, machinery, or furniture in order to operate. Some businesses will require assets such as land, a building, patents, or franchise rights. Major assets that will be used in your business for more than a year are known as "capital assets" and are subject to special treatment under the tax laws. Most importantly, you generally can not deduct the entire cost of acquiring such an asset in the year you acquire it.

One of the goals of accounting is to accurately measure a business's gross income, expenses, and net income (earnings) during a given period of time, usually a year. If a business were allowed to reduce one year's gross income by an expense deduction for the total cost of an item that will be used for several years, the result would be an understatement of earnings in the year the asset was purchased, and an overstatement of earnings during the following years.

It follows that, for "capital assets" (assets that have a useful life of more than one year), the cost must be written off (that is, depreciated or amortized) over more than one year.

Theoretically, the cost of an asset should be deducted over the number of years that the asset will be used, according to the actual drop in value that the asset will suffer each year. At the end of each year, you could subtract all depreciation claimed to date from the cost of the asset, to arrive at the asset's "book value," which would be equal to its market value. At the end of the asset's useful life for the business, any undepreciated portion would represent the salvage value for which the asset could be sold or scrapped.