There are many studies of “multiple solution methods” or “multiple solutions” on mathematical problem solving. In this article, such studies were sorted out according to the types of problems used. The problems were categorized as follows: (A) problems that have multiple solutions mathematically, (B) problems that have multiple solutions due to their contexts, and (C) problems that can be solved by multiple solution methods. In addition, (D) there are comparison studies on correct and incorrect solution methods. The accomplishments of these previous studies as well as their remaining tasks were discussed.