The devastating effects that unsustainable design practices have on the natural world and other species with whom we share this planet have gained widespread awareness and are the driving force behind attempts to develop more sustainable design approaches. These efforts tend to focus on minimising the negative effects that design has on the natural world by reduced material and energy usage. However, the possibility that design may have an active role in mitigating the erosion of biodiversity has only entered the discussion in recent years and remains a marginal activity for design. Following an ongoing paradigm shift calling for the inclusion of a greater diversity of wild animals within human-dominated habitats (as a way of addressing both the erosion of biodiversity and humankind’s alienation from nature), this research proposes that there is a growing need for a design practice capable of responding to the needs of wild animals, while addressing questions of human-animal interaction. In this thesis, Multispecies Design is proposed and developed as a theoretical framework for supporting the shift to more biodiverse human habitats. The research addresses both the physical and socio-cultural requirements of such a shift. Three distinct views define this emerging design approach: recognising animals as clients of design, recognising human-animal interactions as designed experiences and the view of manmade systems as further extensions of ecological systems. The methodological implications of Multispecies Design have been explored in a case study design project concerned with the ecological enhancement of a coastal outfall pipe on a highly frequented beach in Cornwall, UK. The case study explored ways of designing to address the needs of both people and of wild animal species, as well as the interactions between the two groups. It focused on identifying and developing design approaches and tools for studying and representing wild animals in design projects to facilitate their integration into built environments. These tools were further refined in a series of workshops with design and art students carried out during the PhD research. The insights from the practical work, together with the theoretical framework developed alongside them, have led to the development of Principles of Multispecies Design and practical and conceptual Tools for Multispecies Design.