The book value of an asset is its original purchase cost, adjusted for any subsequent changes, such as for impairment or depreciation. Market value is the price that could be obtained by selling an asset on a competitive, open market. There is nearly always a disparity between book value and market value, since the first is a recorded historical cost, and the second is based on the perceived supply and demand for an asset, which can vary constantly.
For example, a company buys a machine for $100,000 and subsequently records depreciation of $20,000 for that machine, resulting in net book value of $80,000. If the company were to then sell the machine at its current market price of $90,000, the business would record a gain on the sale of $10,000.
As indicated by the example, the disparity between book value and market value is recognized at the point of sale of an asset, since the price at which it is sold is the market price, and its net book value is essentially the cost of goods sold. Prior to a sale transaction, there is no reason to account for any differences in value between book value and market value.
One case in which a business can recognize changes in the value of assets is for marketable securities classified as trading securities. A business is required to continually record holding gains and losses on these securities for as long as they are held. In this case, market value is the same as book value.
When the difference between book value and market value is considerable, it can be difficult to place a value on a business, since an appraisal process must be used to adjust the book value of its assets to their market values.
There are situations when the market value of a fixed asset is much higher than book value, such as when the market value of an office building sky rockets due to increased demand. In these situations, there is no way under Generally Accepted Accounting Principles (GAAP) to recognize the gain in a company's accounting records. However, revaluation is allowed under International Financial Reporting Standards (IFRS).