Just-in-Time Teaching (ITT) is a teaching and learning approach that combines the best features of traditional in-class instruction with the communication and resource potential available via the Web. We describe here how ITT can be used to teach biology to undergraduate and graduate level students, both science majors as well as non science majors. A key characteristic of ITT is the creation of a feedback loop between the classroom and the Web using Internet "Warm Up" assignments that are due prior to class time. By examining student responses to Warm Up exercises before class, faculty members can determine the level of understanding, prior knowledge, and misconceptions that students bring to class. Classroom time can then be spent addressing these misconceptions while discussing course content. In class Cooperative Learning exercises reinforce course content in an informal group setting. Other features of ITT, such as "What is Biology Good For," make clear the relevance of specific concepts in biology in society and increase student motivation. Assessment results have been positive, including decreased attrition rates, increases in student attitudes, interactivity, study habits, and cognitive gains in classrooms using ITT. (Contains 2 tables and 7 figures.)