Currently, polymers are widely used as materials for products and parts because of their ease of molding processability and suitable mechanical strength. Measurement of average molecular weight and molecular weight distribution of polymers is indispensable for managing the quality of these products. Size exclusion chromatography (SEC) is widely applied to measurement of polymers because information on average molecular weight and molecular weight distribution can be obtained at the same time by a simple method and precise measurement is also possible. In this chapter, the fundamental principle of SEC and tips on molecular weight measurement of polymers are introduced.