The Peer-Led Team Learning (PLTL) Workshops generally supplement the lecture. PLTL http://www.sci.ccny.cuny.edu/~chemwksp/ can be used in a course with any size enrollment. Under the PLTL model, undergraduate students who have done well in the class previously are recruited and trained as workshop leaders or peer leaders who guide the efforts of a group of six to eight students. These peer-led groups meet weekly (separate from the lecture and the instructor) to work together on problems that are carefully structured to help the students build conceptual understanding and problem-solving skills. There are no answer keys for either the students or the peer-leaders; the emphasis is on learning to find, evaluate, and build confidence in answers. Simultaneously, the workshops and the peer leaders provide a supportive environment that helps each student participate actively in the process of learning science. Thus, PLTL offers a mix of active-learning opportunities for students and a new role for undergraduate peer leaders that is appropriate for their stage of development; PLTL has been used successfully in courses in chemistry, biology, physics, math, computer science, and engineering. In practice, the weekly workshop replaces traditional recitation sections led by graduate teaching assistants or faculty. Although most peer leaders are undergraduates, many graduate students with appropriate training have also worked effectively and enthusiastically in that role.