Professor Grewal’s scholarly interests relate to tax law, administrative law, statutory interpretation, and constitutional law. He has testified in Congress on tax administration issues, and his scholarship in that area has formed the basis for several taxpayer challenges to the Treasury's regulatory authority, including in cases before several United States Circuit Courts of Appeal. The United States Supreme Court cited Professor Grewal in a case involving a complex tax shelter, and the United States Tax Court heavily relied on his regulatory interpretation scholarship in an opinion issued by the full court. Professor Grewal’s research on constitutional issues related to the Trump administration has been cited or discussed in numerous national publications, and he frequently writes about current legal controversies at Notice & Comment, the blog of the Yale Journal on Regulation.