This dissertation addresses three topics on information economics. Generally, information is not perfect or costless as classical economics assumes. Thus, a consumer searches information at his cost or a seller provides information at his cost. First, chapter 2 presents a theoretical model where a consumer searches for local brand information. We show that a national brand providing information has a larger market share. Second, chapter 3 presents a theoretical model where a store randomizes prices and advertises the price changes. We show that at equilibrium the advertising intensity is negatively related to price and price density function is "U" shaped. As advertising costs decrease, average price decreases with more competition. Also as advertising costs decrease from the maximum to zero, price density function changes from monopoly price spike to nonprofit price spike. Thirdly, chapter 4 presents an example where information imperfection is not remedied so information asymmetry remains to cause moral hazard. The deposit insurance rate of a bank is set uniformly regardless of its loan quality because the government cannot discern the quality. Then, a failed bank has higher efficiency in good economic years by spending less on loan monitoring but lending aggressively, but has lower efficiency in difficult years because of its growing non-performing loan. The efficiency of Korean banks between 1990 and 1997 is measured by DEA (Data Envelopment Analysis), and the regression shows that the efficiency of the failed bank is affected by moral hazard. / Graduation date: 2003