Here's what you need to know about the swimmer's visit to Capitol Hill.
What: "Ways to Improve and Strengthen the International Anti-Doping System," a hearing held by the Subcommittee on Oversight and Investigations, which is part of the House of Representatives' Committee on Energy and Commerce
Where: 2123 Rayburn House Office Building, Washington
When: 10:15 a.m.
Video: Webcast at energycommerce.house.gov
Who: Phelps is expected to be joined by fellow witnesses Adam Nelson, 2004 Olympic shot put champion; Travis Tygart, U.S. Anti-Doping Agency chief executive officer; Dr. Richard Budgett, International Olympic Committee medical and scientific director; and Rob Koehler, World Anti-Doping Agency deputy director general. Rep. Tim Murphy, a Republican from Pennsylvania, chairs the subcommittee.
Why: "Throughout my career, I have suspected that some athletes were cheating, and in some cases those suspicions were confirmed. Given all the testing I, and so many others, have been through I have a hard time understanding this," Phelps said in written testimony published ahead of his appearance before Congress. "This whole process takes a toll, but it's absolutely worth it to keep sport clean and fair. I can't adequately describe how frustrating it is to see another athlete break through performance barriers in unrealistic timeframes, knowing what I had to go through to do it.
"If we allow our confidence in fair play to erode, we will undermine the power of sport, and the goals and dreams of future generations," he added. "The time to act is now. We must do what is necessary to ensure the system is fair and reliable, so we can all believe in it."
What else: Phelps' presence in Washington, as well as the uncertain Olympic landscape, raises questions about what else he might say, if anything, beyond his written testimony.
1. How forceful will he be? With U.S. Olympic officials bidding to bring the Summer Games stateside in 2024, Phelps could hurt a potentially lucrative campaign if he encourages Congress to target the IOC as it's deciding between Los Angeles and Paris.
2. Will he get personal? Phelps has been vague about the specific swimmers whose doping he feels sullies the sport. In his testimony, for instance, he wrote: "Throughout my career, I have suspected that some athletes were cheating, and in some cases those suspicions were confirmed." Phelps is a public-relations pro, so don't expect him to name names, even if someone asks him about Yulia Efimova.