For the purposes of this study, we have broadly categorised the results of the activities of botanic gardens into economic, social and environmental impacts. Wherever possible, this Technical Review has highlighted case studies where impact evaluation studies have been carried out by objective, third parties – usually auditors, consultants or academics. The results of such studies are useful to policymakers and funders because they enable decision makers to weigh up the cost: benefit ratio or return on investment associated with particular activities carried out by botanic gardens. Such studies are useful to botanic garden leaders for the same reasons. They can be used to persuade funders and influencers that botanic gardens are institutions that are worth supporting. For small gardens that are unable to invest in studies of this kind, the principles and case studies illustrated in this Review may be useful in general terms by persuading supporters that botanic gardens are sound investments even if impacts can’t be measured precisely for a particular institution.