Fault diagnosability is the fundamental for fault diagnosis. The knowledge of achievable diagnosability performance can provide the theoretical guidance for the development of diagnostic algorithms and the optimization of sensor placement. The concept and evaluation process of diagnosability are firstly introduced. Then existing fault diagnosability evaluation methods for the continuous systems are categorized into three types, i.e., methods based on the quantitative model, the qualitative model, and the statistical data. The characteristics of each method are summarized, and comparison of qualitative and quantitative evaluation is presented. Finally, several potential research tendencies are discussed.