This paper is devoted to the fragility analysis of PI controllers. Two different approaches are proposed to estimate the fragility of a given PI controller; they can be used independently of each other. The first approach is based on the ideas of ellipsoidal estimation and the technique of linear matrix inequalities. Within the second approach, involving the so-called parameter loop opening, the parameter under study is “extracted” from the plant–controller system, forming a fictitious control loop; after opening, this loop is analyzed by classical frequency-domain methods (the Nyquist criterion and D-partition). The features of both fragility analysis methods are described, and numerical examples are provided.