We are currently experiencing unprecedented environmental changes driven by anthropogenic activities with consequences that include soil erosion, nutrient enrichment, population and species extinctions, and species invasions (Corlett, 2016). These rapid changes generate uncertainties that may compromise the goals and priorities of conservation and management efforts (Wiens and Hobbs, 2015), including rewilding attempts. Some conservationists, including rewilding advocates, subscribe to the ideal that natural processes should be allowed to take their course without human intervention. Others believe that such an approach is too risky so it is more appropriate to actively manage nature (Corlett, 2016). However, rewilding outcomes may become more unpredictable because of uncertainties in future conditions (e.g. climate change, land conversion) and increased frequency of extreme events. In this chapter, we focus on how trophic and passive rewilding initiatives may intensify the risk of unwanted ecological effects. We do not address potential economic and societal implications of rewilding initiatives because there are covered in other chapters (see Chapters 8, 9 and 19). In addition, we show that biological communities can be understood only by considering their evolutionary history, and we warn that ignoring this point in rewilding projects could ultimately risk failure.