This chapter sets out two popular psychological theories that describe the importance of illness cognitions and beliefs for predicting health behaviours and health outcomes. The evidence for these two theories is reviewed along with directions for future research. Illness cognitions and beliefs are examined from a lifespan perspective, describing the views of children, young adults, and older adults. Similarities and differences in beliefs and cognitions based on ethnicity are described, both from a US and from a global perspective. Finally, an overview of interventions to change illness cognitions and health beliefs is provided along with evidence of their effects.