In this lecture, the students will learn intermediate-level microeconomics. Assuming that introductory-level microeconomics has been already studied, the lecture focuses on the foundations for advanced economic theory. The lecture begins with a review of the scope of microeconomics. The analysis of economic agents engaging in economic activities follows. The model of consumer behavior is presented, which facilitates the understanding of the concept of demand. The lecture moves on to the analysis of firms, another type of players in economic systems, to introduce the concept of supply. Economic transactions are basically the result of balance of supply and demand. A key conception is “equilibrium.” Its implications to social welfare are discussed in detail.