Astronomy is a popular subject for non-science majors in the United States, often representing a last formal exposure to science. Nationwide, more than half of all college students take at least one class online each year. In addition, there has been a rapid growth in Massive Open Online Classes (MOOCs), where adult learners take an online class for enrichment rather than for credit towards a degree. For both formal and informal learners, online course delivery is becoming increasingly important, and the resources for instructors have not kept up with this rapid change. This book aims to fill that need, with advice on all the tools and resources that are suitable for online classes. The book's purpose is to bring astronomy instructors up to speed on the best ways to create and teach an online astronomy class, for traditional college students and for distributed audiences of lifelong learners. Instructors of these courses will see articles on the online use of real and virtual telescopes, simulations and applets, and tools that adapt to the learner. Each chapter is written by an academic who is adept in teaching online classes to diverse audiences.