Active learning leads to substantially better learning than occurs with traditional lecturing. This talk reviews the nature of active learning and explains why it is so effective. The talk focuses on fundamental principles drawn from the science of learning; these principles range from Deep Processing (the more mental effort one expends when processing information, the more likely it is to be remembered) to Chunking (humans can only absorb about four organized units at the same time, but each of those units can have up to four parts) to Deliberate Practice (practice is most effective when one uses feedback to focus on the most difficult aspects of the knowledge or skill). We not only consider the principles, but also see how to apply them to teaching—particularly when teaching online, both in synchronous settings (i.e., live, as occurs on Google Meet, Webex or Zoom) and in asynchronous settings (i.e., when the student and instructor are not present at the same time, and students learn largely at their own pace). The talk practices what it preaches by relying on demonstrations and illustrations of key points.