Mitigation measures have been designed and implemented worldwide to counteract the effects of roads and traffic on wildlife, but the effectiveness of many types of mitigation at achieving specific goals remains unknown. In this chapter, we summarize the evolution of the field by framing the discussion of effectiveness in relation to no-net-loss policies. We highlight the need for more and better studies of mitigation effectiveness at the population level and for a stronger focus on mitigating the mechanisms underlying the effects of roads on populations. Evaluations of mitigation effectiveness need to measure variables related to the outcomes of concern, which are linked to the road impacts targeted by mitigation. There is still much to learn about mitigation effectiveness, and we should answer questions such as how many animals in a specific population need to cross to maintain population viability; how much movement is enough to mitigate the barrier effect at the population level; and what level of roadkill is sustainable from a population viability perspective. We recommend five priorities for future research and practice: improve environmental impact assessments as an evidence base for mitigation, apply more robust study designs when evaluating mitigation measures, better define what effective mitigation means at the population level, study the effectiveness of combinations of mitigation measures at different spatial and temporal scales, and make transportation agencies accountable. An agenda for improved evaluation of mitigation effectiveness can be implemented by combining efforts among transportation and environment agencies, practitioners, and researchers in a systematic framework.