Growing concern over the potentially devastating impacts of climate change on biodiversity and food security, considered together with the growing world population, means that taking action to conserve crop wild relative (CWR) diversity is no longer an option-it is an urgent priority. Crop wild relatives are species closely related to crops, including their progenitors, which have the potential to contribute beneficial traits for crop improvement, such as biotic and abiotic resistances, leading to improved yield and stability. Having already made major contributions to crop improvement in the 20th century, CWR are recognized as acritical resource to sustain global food security; therefore, their systematic conservation is imperative. However, extending their conservation and promoting more systematic exploitation is hindered by a lack of understanding of their current and potential value, their diversity, and practically how they might be conserved. Therefore, the aim of this paper is to (i) demonstrate the current and potential use of CWR in crop improvement, (ii) estimate how many CWR species exist and how many are a global priority for active conservation, and (iii) describe how a global network for the in situ conservation of CWR might be established that could help to underpin future food security.