Peer Instruction was developed by Eric Mazur at Harvard in the 1990s and is now the most widely implemented PER-based teaching method, used in hundreds of high school and college classrooms throughout the world. Mazur stumbled onto the method during a review session when he could see by the confused looks on his students’ faces that they did not understand the concept he was trying to teach, but he did not know how to explain it any better than he had already done. He asked the students to turn to their neighbors and discuss it, and the room erupted into discussion. Many instructors have adopted and adapted this method, and many variations that do not follow the precise instructions given by Mazur are often referred to as “peer instruction.” We use capitalized “Peer Instruction” to refer to the specific technique laid out by Mazur, and lower-case “peer instruction” to refer to any number of variations that follow the same basic philosophy.
You can see Mazur's personal story of the development of Peer Instruction on YouTube: