For most of U.S. history, presidents have issued signing statements to comment on bills being signed into law. These statements often are hortatory and comment on the merits of the new law. In recent decades, presidents also have used signing statements to indicate portions of laws that they consider unconstitutional. Pointing out such parts of new statutes is not a problem, but indicating that the president may not execute part of the law is problematic. President George W. Bush used signing statements in an aggressive way to imply that he might not faithfully execute more than 1,000 provisions of statutes that he signed into law. This essay argues that this practice undermines the rule of law and threatens the separation of powers system.