This chapter explores a number of aspects of mark-to-market (MTM) accounting, to better understand the effects of a change in accounting method. Corporate plan sponsors have an ongoing management challenge with defined benefit (DB) pensions. Among the many issues is how to properly account for their associated assets and liabilities, and to provide clarity regarding the plan itself while not diluting transparency into the underlying business performance of the corporate plan sponsors. Traditional generally accepted accounting principles (GAAP) for pensions is an overly complex mechanism, attempting to balance these two often conflicting goals. There is an alternative approach: MTM accounting.