This chapter aims to demonstrate that the natural environment is a key determinant of human health. Natural environments are defined as organic environments where the majority of ecosystem processes are present (e.g. the cycle of life and death, reproduction, relationships between species). Including a spectrum of different environments ranging from wilderness areas in National Parks to farms, and human constructed gardens, 'natural' does not necessarily mean unaffected by human intervention. 'Nature' can also refer to any single element of the natural environment (such as plants, animals, soil and water), including domestic and companion animals as well as cultivated plants. The term 'natural environment' differs from the more usual use of the term 'environment' in that it can be a setting in itself, or it can be an element of other settings, including our homes, workplaces and schools.