Rewilding — the proposed restoration of ecosystems through the (re-)introduction of species — is seen by many as a way to stem the loss of biodiversity and the functions and services that biodiversity provides to humanity. In addition, rewilding might lead to increased public engagement and enthusiasm for biodiversity. But what exactly is rewilding, and is it based on sound ecological understanding? Here, we show that there is a worrying lack of consensus about what rewilding is and what it isn’t, which jeopardizes a clearer account of rewilding’s aims, benefits and potential consequences. We also point out that scientific support for the main ecological assumptions behind rewilding, such as top-down control of ecosystems, is limited. Moreover, ecological systems are dynamic and ever-evolving, which makes it challenging to predict the consequences of introducing novel species. We also present examples of introductions or re-introductions that have failed, provoking unexpected negative consequences, and highlight that the control and extirpation of individuals of failed translocations has been shown to be extremely challenging and economically costly. Some of rewilding’s loudest proponents might argue that we are advocating doing nothing instead, but we are not; we are only advocating caution and an increased understanding and awareness of what is unknown about rewilding, and what its potential outputs, especially ecological consequences, might be.
