When assets are recorded a company's balance sheet, they are valued at historical cost (what was paid for the asset), less any accumulated depreciation or amortization if appl
…icable. This holds true even if the market value of the asset is considerably more than what the company paid for it. However, if the market value of a company's assets drops significantly below the asset's historical cost, then it sometimes becomes necessary to revalue the asset at the lower market value. This revaluation is called impairment. When it is appropriate to impair an asset depends on the type of asset in question. The difference between the current book value of the asset, and the value of the asset after impairment, is your impairment expense (cost). ( Full Answer )