This thesis is about the relationship between radical environmental activists and nature. It investigates whether, for forest and whaling activists, nature is experienced as an active, as opposed to passive, participant in the construction and shaping of their identity and activism. Two research questions guide the investigation: what are radical environmental activists' perspectives and lived experiences of nature, and what identity and meaning-making processes are involved in the relational dynamics between these activists and the nature they are defending? The concepts 'nature', 'self-identity' and 'more-than-human agency' are developed into an analytical framework to support the investigation. A phenomenological perspective guides the inquiry's focus on the research participants' lived experiences of defending nature, their changing self-identities and the ways they construct meaning about their lifeworlds. Forest activists engaged in direct action campaigns designed to prevent clear-felling of old growth forests in Tasmania, and Sea Shepherd Conservation Society activists undertaking campaigns in the Southern Ocean to protect whales, participated in in depth qualitative interviews. A thematic analysis was employed that aims to uncover the phenomenological themes, or experiential elements, of the participants' experiences. The inquiry's findings contribute to environmentalism scholarship and the study of nature-human relationships. They also demonstrate the need to appreciate the role of nature as an active contributor to activist self-identity and culture.