In this course, we will study how consumers and firms make decisions and how they interact in the markets. We will introduce key economic concepts and the basic tools to analyze the behaviors in the markets and in the society. In this semester, we will cover the topics including consumer's behavior, firm theory, perfect competition, market equilibrium and welfare economics. In the next semester, we will focus on monopoly, oligopoly, externality and public goods, information and uncertainty, and game theory.