Impeachment was reinvented in seventeenth-century England and put to prodigious use in conflicts between crown and parliament. Judicial in form, but political in practice, impeachments were arguably judicial failures yet politically they were immensely powerful. Failures because very few impeachments ran their full course; successes because royal ministers were nonetheless brought down. Impeachments provided a spectacular political theatre of public outrage and retribution; but they were not a practical means for long-term, settled oversight of executive power. This chapter explores the evolution of impeachment proceedings; their rhetoric and implied political theory; the expansion of the concept of treason, from sedition against the monarch to subversion of the constitution; the legal and political impediments that often stymied proceedings; and the relationship between impeachment and attainder. There is a coda on impeachment's export to Ireland and colonial America.