Churn is one of the main problems faced by all P2P networks. This paper surveys the existing theories and methods about Churn. On the basis of the generation mechanism of Churn, this study summarizes the major steps of the Churn problem solution: precise measurement of Churn, analysis of the impact of Churn on P2P performance, and provision of specific strategies to handle Churn. Under each step, existing problems, solutions, and the newest recent research are explored. This paper also discusses the prospects of future studies.