Impairment and depreciation are two related concepts used to describe the gradual decline in value of assets over time. Depreciation is a noncash expense which reduces the value of an asset each period, whereas impairment occurs when there are significant changes which lead to an expectation that the asset may be worth less than its carrying value. The reason for this difference is because impairment is triggered by an event or change, such as obsolescence, while depreciation is part of the natural aging of an asset. Impairment marks a shift in the expected cash flows associated with the asset, while depreciation is simply a mechanical process which slowly reduces the value of an asset.