Simulation is an important methodology in bridging the gap between theory and practice in medical education. Simulation-based education (SBE) provides a structured, learner-centred environment in which novice, intermediate, and advanced practitioners can learn or practise skills without causing harm to patients. A range of systematic reviews indicate that simulation-based medical education can improve knowledge and skills and, increasingly, improve patient outcomes. The advantages and benefits of simulation are reflected in its increasing global use as a learning methodology in medical school and in continuing medical education. This chapter provides an introduction to SBE. It discusses the range of simulation modalities and the drivers for the increasing use of simulation within medical education. Relevant learning theories are described, alongside their implications for simulation practice and a summary of the evidence supporting SBE. The major focus of this chapter is in describing how to design SBE, with an in-depth discussion of mastery learning, an approach underpinned by both theory and evidence. We introduce the mastery learning instructional design processes, which are well suited to SBE, as simulation environments can be designed to provide incremental levels of challenge in a controlled environment with significant opportunities for feedback. SBE also provides an opportunity to introduce social and affective elements of practice, while still supporting learners, although there are particular limitations to simulation environments in this regard. We provide two case studies, one undergraduate and one postgraduate, to illustrate the ideas and principles discussed within this chapter. Logistical and organizational issues form a significant part of the barriers to implementing SBE, so the chapter closes with a short discussion of these issues, which can present real barriers to the use of SBE within medical education at all levels.