"Accountable Talk" begins with students thinking out loud about a complex problem that requires collaboration: noticing something about the problem, questioning a surprising finding, or articulating, explaining, and reflecting upon their own reasoning. The teacher works to elicit a range of ideas, which may be incomplete. With teacher guidance, other students take up their classmates' statements: building on, challenging, or clarifying a claim (including a teacher's claim); posing questions; reasoning about a proposed solution; or offering a counter claim or an alternate explanation. There are clear standards for what counts as a good discussion, often described as the "three accountabilities": (1) accountability to knowledge (getting the facts right even if it is a struggle to find the right wording); (2) accountability to reasoning (providing a rational justification for a claim); and (3) accountability to community (showing respect for the ideas and feelings of classmates). Overall, the teacher's goal is to sustain a "teacher-led" but "student-owned" process of shared reasoning that ultimately leads to a more fully developed, evidence-backed conclusion, solution, or explanation. This booklet presents eight principles that address the "why" and the "how" of "Accountable Talk."