It is commonly believed that it is important for safe evacuation in a building fire to arrange for two or more escape routes in different directions. Various provisions are prescribed in building and fire codes of many countries to implement this concept in buildings. The requirement of two or more exits from a room is one of such provisions. However, the specifications for requiring two or more exits vary significantly from one country to another, and the level of safety assured by the provisions is not clear. In this chapter, the meaning of the requirement of two or more exits is considered from the viewpoint of the expected number of occupants unable to escape, and a criterion for the number of required exits is proposed as an alternative to the existing provisions. The proposed criterion is basically the same as existing provisions in terms of the average level of safety assured, but allows an assessment of the effect of the number of exits on safety more explicitly.