Arguing for the federal government will be the Acting Solicitor General of the United States, Neal Kumar Katyal; arguing for the states will be Paul Clement, a former Solicitor General in the Bush Administration.
This case was originally brought by the Attorney General of Florida; the other 25 states have joined over the 15 months.
Legal experts see this case - and the over two dozen others now working their way through the legal system - getting to the U.S. Supreme Court in one form or another over the next six to nine months.
These legal challenges may well represent the best chance that opponents have to overturn the Obama health law, as critics don't have enough votes to repeal the plan in Congress.
While Republicans have vowed to "repeal and replace" the Obama health law, GOP lawmakers in the House have yet to produce their own plan for votes in various committees, let alone on the floor of the House.
The U.S. Senate has twice rejected a repeal plan on party line votes.