This lecture explores market imperfections in the context of land and real estate economics, discussing how the market equilibrium may not always lead to the best outcome for society due to external costs and benefits. It delves into cases where social costs differ from private costs, leading to the need for taxes or subsidies to internalize externalities. The lecture also addresses scenarios where social values conflict with private values, requiring interventions to align market outcomes with societal goals. Overall, it emphasizes the importance of considering collective interests alongside individual pursuits in real estate markets.