This thesis investigates the role of artefacts in relation to human agency and design within a cognitive semiotics framework. It deals with questions such as What is agency? What are artefacts and how does agency relate to them? What kind of intentions are involved in the activity of designing? What is human-specific with respect to agency and design? How can the origins and evolution of design be explained? Addressing these questions, the thesis proposes a layered model of agency for explaining the relations between different grades of agentive complexity. The model is also useful for empirical studies of agency such as those of neurosci- ence. The thesis contends that the activity of design is a key feature of human agency. Thus, it delves into the cognitive processes of design, proposing the notion of enhanced agency as the prosthetic incorporation of artefacts into the agentive capabilities of the agents. The thesis also explores the origins and evolution of design and proposes a stage-based model in which the progressive complexity of the artificial world is parallel to the increasing complexity of enhanced human agency. This thesis rejects human exceptionalism which places humans as ontologi- cally unique and biologically discontinuous with the rest of the living world. At the same time, it recognizes that only human agents have the power to stop the current environmental obliteration. This can be done by recognizing the potential of human agency, instead of diluting it into abstract networks; highlighting its differences and similarities instead of equating human agency with inert matter or anthropomorphizing the agency of other animals. This also highlights the moral responsibilities of human agency.