This chapter discusses the relationships between psychological factors and disease progression. Psychological correlates discussed include stress and depression, the influence of social relations, coping responses, and emotional repression, as well as the influence of socioeconomic status. The relevance of the theory of allostasis and the concept of allostatic load and the importance of including a developmental perspective are described. The neuroendocrine-immune correlates that are discussed in the chapter are cortisol and its relationship to circadian rhythmicity and corresponding T helper 1/T helper 2 cell (Th1/Th2) immune balance, CD4 activity (T helper cell marker), and functional and enumerative activity of natural killer (NK) cells. The enhancement of immune function is most beneficial for immunocompromised populations while such an enhancement in those whose immune functioning is already optimum may be impossible or even harmful. The diseases presented in this chapter meet this criterion because cancer is associated with immunosuppression both directly through metabolic, metastatic, and other mechanisms and indirectly through the effects of chemotherapy and radiotherapy, and HIV/AIDS is a progressive assault on the immune system.