Modal logic is an important extension of classical logic. In philosophy, we could view it as a technical tool to formally investigate many important philosophical concepts, such as knowledge, obligation, time, counterfactual, and so on. This course is purported to introduce the concepts and techniques in modal logic such that students can have the basic knowledge to work in the future in diverse branches in philosophy, and also in other areas such as computer science and game theory, where modal logic is also useful tool.