Taxi service is an indispensable part of public transport in modern cities. The taxi system is operated by a large number of self-controlled drivers lacking of centralized scheduling and control, which makes it inefficient, difficult to analyze and optimize. It is thus important to take into account taxi drivers' strategic behavior in order to optimize taxi systems' efficiency. This paper reviews existing taxi system researches for modeling taxi system dynamics, introduces the taxi system efficiency optimization problem, and presents a game theoretic approach for optimizing the efficiency of taxi systems. Challenges and open issues in the taxi system efficiency optimization problem are also discussed.