Market participants in the U.S. have spent the better part of the year adjusting to new rules as mandatory trading of a range of OTC derivatives on multilateral trading platforms known as Swap Execution Facilities (SEFs) finally became a reality. This paper briefly summarizes the introduction of the SEF mandatory trading obligation and the made-available-to-trade (MAT) determinations that spell out the derivatives that must now be traded on SEFs. More importantly, we take a closer look at how mandatory SEF trading has impacted volumes of interest rate swaps (IRS) traded on these platforms.