The most recognized definition of the circular economy is that it is a restorative and regenerative economy. Despite the wide use and importance attributed to the concepts of "restoration" and "regeneration," they are rarely defined or explained in the circular economy literature. In this context, this study critically examines the two terms, while providing guidance on their future utilization and development. Specifically, the study investigates the origin of the concepts, their adoption in frameworks that anticipated the idea of the circular economy, and their connotations in the circular economy literature. The examination supports the need for clear and distinct definitions, combined with precision in usage. From a review of the literature, restoration is a better-defined concept than regeneration, although it needs conceptual re-enforcement relative to the biological/ecological aspects of the circular economy. This study suggests looking in the direction of restoration ecology, a well-established branch of ecological research. Conversely, regeneration is a symbolic/evocative term with little practical application in the context of circular systems except in the case of certain agricultural practices. Until new conceptual developments intervene, regeneration does not seem to be applicable to the economy as a whole and because of this, might be abandoned as a guiding principle of the circular economy. Unlike regeneration, restoration can be considered a core principle because it has widespread application and can be a point of reference for circular applications. This does not preclude the possibility that other concepts may be needed to augment restoration. K E Y W O R D S biological and technical cycles, circular economy, circular economy guiding principles, industrial ecology, restorative and regenerative concepts 1 INTRODUCTION In 2012, the Ellen MacArthur Foundation (EMF) introduced the concept of a circular economy (CE) as "an industrial economy that is restorative or regenerative by intention and design" (EMF, 2013, 7), and later as "restorative and regenerative by design and aims" (EMF, 2015a, 2). Since then, the definition of the CE as restorative and regenerative has come into common use, or at least has reverberated thousands of times throughout both academic and non-academic literature. This definition is by far the most employed one in CE studies (Kirchherr, Reike, & Hekkert, 2017). To give a flavor of this, up to the end of 2019, Google Scholar counts over 700 entries for the combination of "circular economy" and "restorative and regenerative" and almost 400 for "circular economy" and "restorative or regenerative." The majority of quotations are verbatim from EMF sources, although some authors provide a modified or slightly expanded version of the EMF definition (e.g.