This chapter presents an overview of the concept of toughness, which at the abstract level is about the harmony of physiological systems, and more concretely is about how the body influences the mind. Toughness theory begins with the recognition that there is a “training effect” for neuroendocrine systems. Following a review of the characteristics of interventions and training programs that can promote toughness, the authors present a model in which the effects of toughness are mediated by neuroendocrine systems such as the pituitary-adrenal-cortical system and the central nervous system. The elements of toughness (e.g., having a greater capacity for arousal and energy when needed) are proposed to promote positive outcomes by facilitating the use of adaptive coping strategies and improving emotional stability. Toughness therefore appears to be a promising concept within positive psychology in that it helps to explain how the dynamic interactions between psychological and somatic processes can promote positive outcomes.