On 12 June 2007, the IRS published long-anticipated final safe harbour rules for the valuation of securities and commodities for Sec. 475 mark-to-market accounting purposes. This article explores the rule-making process by examining the original stated objectives of the IRS and the Treasury Department, and considers industry input regarding the business and accounting practices of the taxpayers involved. The final regulations implementing the original objectives, as refined by public comments, are discussed as well.