Information Systems (IS) interoperability in Public Administration (PA) is a main goal and a major challenge for PA professionals. Achieving interoperability among IS that are technologically disparate and that exist in different organizational contexts is a complex task, being affected by multiple aspects, not yet satisfactorily known and characterized. The aim of this paper is to unveil the forces that influence IS interoperability initiatives in PA. The inquiry was inspired by Lewin’s Field Theory. The data generation process was based on a Delphi study involving 55 experts from PA, IS/IT industry, and academy. A set of 31 forces were identified and ranked based on the level of importance they assume in IS interoperability initiatives. Thirty eight propositions describing restraining and driving influences were also formulated, as well as 24 propositions about forces’ configuration in the current context of Portuguese PA that represent the specific constellation of forces acting in IS interoperability initiatives in that country. The results of this study provide an understanding of the complex of forces acting in IS interoperability, contributing to improve the study, management, and implementation of these initiatives and, consequently, to the establishment of a PA with more adequate, sustained, and sustainable levels of interoperability.