This chapter analyzes theoretical conceptions of health and disease for plants and animals. Compared to human health, the discussion of these concepts is sparse, although animal health has received more treatment than plant health. The concept of disease seems to be taken for granted, and few attempts have been made to make a classification of specific types of diseases. The main emphasis in this chapter will be on the contemporary debate on health definitions in these fields. Although the theoretical discussion on these issues has so far been limited, a categorization of different kinds of health definitions has been presented for animals and plants. The four health categories are health as balance, as biological function, as well-being, and as ability. Similarities and differences between health concepts for humans, animals, and plants are also discussed to see whether there is a possibility to have a universal definition for plants, animals, and humans. This will have implications for One Health approaches.