Fidelity concerns the extent to which a specific evaluation sufficiently incorporates the core characteristics of the overall approach to justify labeling that evaluation by its designated name. Fidelity has traditionally meant implementing a model in exactly the same way each time following the prescribed steps and procedures. The essential principles of developmental evaluation (DE), in contrast, provide high-inference sensitizing guidance that must be interpreted and applied contextually. In lieu of operationalizing DE fidelity criteria, I suggest addressing the degree of manifest sensitivity to essential principles. Principles as sensitizing concepts replace operational rules. This means that sensitivity to essential DE principles should be explicitly and contextually manifest in both processes and outcomes, in both design and use of findings. Eight essential principles of DE are identified and explained. Finally, 10 threats to evaluation model fidelity and/or degree of manifest sensitivity are identified with ways to mitigate those threats.