Abstract: Health engineering was studied in terms of Pareto principle in this paper. A function was classified into a normal function which optimum performance was maintained by its negative feedback with its respective functional fitness so that it was in its function-specific homeostasis (FSH), and a dysfunctional function which was far from its FSH. A normal function-specific stress to disrupt the FSH of a normal function was classified into a successful stress which optimum performance was maintained by its negative feedback so that the functional fitness was upgraded and a chronic stress which was far from its negative feedback. Health was a state of an organism all of which essential functions and key non-essential functions were normal or a stress of which normal function was successful. Suboptimal health was a state of a disease-free organism only some of which key non-essential functions were dysfunctional in comparison with its healthy state. Disease was a state of an organism which was not in both health and suboptimal health. The health diagnosis, functional training and medical treatments were then discussed in terms of traditional Chinese medicine and modern medicine.