The aggravation of ecological situations, and their epidemiological significance as well as awareness of the nature social value influence the level of ecological anxiety within society and that of both constructive and destructive ecological activity. Ecological nihilism is transformed into ecological activism making the state follow public demand for a favorable environment and ecological policy meeting the requirements of social security. Ecologization and radicalization of civic activism are also conditioned by the interaction of ecosystem problems; politicization trends, manipulation of environmental situations at the global and local levels; processes of informatization, individualization, and alarmism. Proponents of eco-activism initiate social and political movements, transformations, discourses, and actions, interdisciplinary assessment of which requires systematization and generalization. And the focus of this article is civic activism conceptualization in terms of greening and radicalization, based on the systematization of its interdisciplinary scientific definitions, explanations of its diverse manifestations. The scientific methods used are the authors' interpretative analysis of academic texts; comparative and derivative analysis of sociological empirical research results. Having considered interdisciplinary attributes and characteristics of civic activism forms, the authors propose their own interpretation of environmental radicalism. Environmental radicalism is a type of civic activity coherent with environmental problems, human rights-based approach and political radicalism, both non-violent and aggressive delinquent actions that pose public danger being used. An utmost form of environmentalism is eco-extremism, which risks for society are much higher due to the use of illegitimate and illegal violence against ordinary citizens and traditional institutions.