Most people think of a teacher leader as someone who takes on additional roles outside the classroom. But in this article, Michelle Collay argues that all effective teaching requires leadership. Using examples from classroom teachers with whom she has worked, Collay describes four dimensions of powerful classroom-based leadership: (1) teaching well by drawing on professional knowledge to respond to the complex relationship challenges that affect learning; (2) finding creative ways to collaborate with both colleagues and parents; (3) conducting classroom-based inquiry, both through informal observation and through formal, structured research; and (4) using partnerships with outside organizations to support student learning. Both school leaders and teachers themselves, writes Collay, should become more aware of the powerful leadership roles played by teachers both within and beyond their classrooms.