The National Center for Atmospheric Research (NCAR) offers the Climate Discovery online course sequence for middle and high school teachers. The sequence is composed of three courses - Introduction to Earth's Climate, Earth System Science: A Climate Change Perspective, and Understanding Climate Change Today. Each course in the sequence is 6 weeks in duration, and includes readings, interactives, discussion fora, and assignments that provide opportunities to assess participant progress. Thanks to the online format of the course sequence, which developed as an outgrowth of a previous in-person workshop held annually at NCAR during the summer for 20 teachers, we have been able to provide professional development to more teachers than we would have otherwise been able to reach continuing our in person workshop series. Interaction among workshop participants and with instructors has been particularly extensive, and participants appear to continue use of course materials after the course. Feedback from participants has been extremely positive, with a significant fraction of participants enrolling in more than one course of the sequence.