Co-teaching involves two or more certified teachers sharing instructional responsibilities for a single group of students in a classroom. There are several different models of co-teaching, including one teach one observe where one teacher leads instruction while the other observes students, parallel teaching where teachers split the class into two groups and teach the same content simultaneously, and alternative teaching where one teacher instructs the large group while the other works with a smaller group. The most complex model is team teaching where both teachers deliver the same instruction at the same time.