Medical schools have been slowly adopting online learning into pedagogical methods for more than a decade. While some medical educators are reluctant to accept these changes, the ongoing coronavirus disease 2019 (COVID-19) pandemic poses a threat to the delivery of traditional medical education, which has accelerated the inevitable implementation of online learning. This sudden change may be a new challenge to medical educators who are new to this territory. Therefore, this review aims to provide foundational concepts of online learning and practical guidelines in the context of medical education. The authors first identify three foundational concepts, which are transactional distance, presence, and independent learners. In online learning, transactional distance, determined by dialogue and structure, becomes more important than physical distance. Furthermore, effective and successful online learning requires the achievement and accommodation of cognitive, social, and teaching presences. It is also crucial to recognize learners not as passive recipients of information predefined by a teacher, but as active, capable, and independent individuals. The authors, then, discuss the practical guidelines for designing an effective online curriculum. Five online pedagogical guidelines are laid out in this review: design structures and flows to embrace experiential learning, accommodate both synchronous and asynchronous learning, design/facilitate interactions, promote practice opportunities, and promote a learning community. By understanding the foundational concepts and applying these guidelines, the adoption of online learning in the medical school may supplement the traditional medical education or even provide additional benefits in the new normal after the COVID-19 pandemic.