Editors' Summary: The Supreme Court's recent Sweet Home decision validated the U.S. Fish and Wildlife Service's regulation defining "harm" under the ESA to include habitat modification. The decision leaves private landowners facing uncertainty regarding what types of actions they may take on their land without causing "harm" to a listed species and thereby committing a "take" of the species in violation of the Act. The authors examine the statutory "take" prohibition and the regulatory provisions further defining "take" and "harm." They then discuss the history of the Sweet Home case and the Sweet Home decision itself. Next, they describe the scope of the "harm" regulation in light of Sweet Home and several other decisions under the Act. They conclude that these decisions have substantially narrowed the extent to which habitat modification on private land may constitute a "take" under the ESA, but that substantial uncertainty remains.