The CDC has reported that 61% of adults have been exposed to at least one adverse experience and 16% have experienced at least four adversities in childhood alone (Center for Disease Control and Prevention [CDC], 2024). Researchers have investigated how adversities across the lifespan, including childhood (e.g., Schalinski et al. (2016), adulthood (e.g., Mersky et al., 2021), and a combination of the two (e.g., Lamoureux-Lamarche & Vasiliadis, 2017), influence mental health. Competing theoretical models exist to explain the relationship between childhood and adulthood adversity in the prediction of mental health outcomes, including the cumulative risk model (Evans & Kim, 2010) and the sensitization model (Post, 1992). Although evidence exists for both models, findings on the cumulative risk model are more consistent than those for the sensitization model. Previous studies have suggested that the key question is not whether childhood adversity moderates the relationship between adult adversity and mental health, but when it does (Simons et al., 2019). Through a systematic review, Chapter 1 synthesizes the evidence for these two models. Chapter 2 subsequently examines whether childhood and adulthood adversity are cumulative or interactive predict post-traumatic stress symptoms (PTSS) and subjective well-being (SWB). Chapter 2 also explores whether specific subtypes of childhood adversity interact differently with adulthood adversity in the prediction of PTSS and SWB. A sample of U.S. adults completed measures on childhood and adulthood adversity, PTSS, and SWB. Hierarchical regressions were utilized to examine whether childhood and adulthood adversity predicted PTSS and SWB in line with the cumulative risk or stress sensitization model. Further, the potential moderating effects of four subtypes of childhood adversity on the relationship between adulthood adversity and PTSS and SWB were explored. Findings revealed that childhood and adulthood adversity had a cumulative effect on PTSS but interacted in the prediction of SWB. Additionally, the results of exploratory analyses suggested that none of the childhood adversity subtypes moderated the relationship between adulthood adversity and PTSS. However, one subtype of childhood adversity moderated the relationship between adult adversity and SWB, whereas the other three did not. Interpretations and clinical implications are discussed.