Nature-based solutions are used by climate policies because they are societal solutions to mitigation and adaptation (that have a human product). Therefore, if humans want to diminish the adverse effects of climate change, then a framework and a theory on nature-based solutions based on public policy, law, science, and societal needs are necessary because these are the tools to developing solutions, and the lack of knowledge on the effectiveness of these solutions is pointed out by commentaries on nature-based solutions. The paper’s objective is to develop a framework and a theory on nature-based solutions by analyzing the theoretical concepts of nature-based solutions vis-á-vis climate change, particularly adaptation and mitigation as concepts to be compared with the concept of nature-based solutions, and to analyze cases related to ecological corridors, rewilding, and other cases based on private and/or public efforts toward diminishing the effects by mitigation or to help people and biodiversity by adaptation. This chapter proposes that nature-based solutions may advance extant conservation in which mitigation and adaptation are key driving forces. A methodological approach based on a qualitative methodology from case studies, documents, studies will be considered.