This thesis considered the influence and implications of age differences and judgmental processes on illness cognition. The overall aim of the thesis was to document an account of some of the strategies and shortcomings of the health-related judgments of older and younger adults. To this end the methodologies employed were drawn heavily from judgment and decision making research paradigms well established in Behavioural Decision Research. Consequently, nine experiments are reported which were designed to cover a range of inferential tasks that confront the layperson in the context of health-related judgment. These experiments divided roughly into three types of inferential tasks: I Subjective Risk Assessment (Experiments 1-4); II Covariation Judgment (Experiments 5 and 6); and III Hypothesis Testing. (Experiments 7-9). Based on a series of classic experiments conducted by Lichtenstein et al. (1978) the first three experiments showed that the views of the older (60-79 yrs) and younger adults (20-29 yrs) were similar in terms of those health risks which they considered to be more or less common than others; both older and younger adults systematically underestimated the occurrence of high frequency causes of death and overestimated the occurrence of low frequency causes of death. By considering factors that may underlie older (60-79 yrs) and younger people's (20-29 yrs) judgments of their comparative risk, Experiment 4 supported the findings of previous research which has demonstrated unrealistic optimism to negative life-events (Weinstein, 1980, 1982). Furthermore, age differences did emerge between the older and younger adults in the nature of the health-related events that were most likely to evoke unrealistic optimism. In relation to the second type of inferential task, Experiments 5 and 6 aimed to establish the extent to which older and younger adults' pre-existing theories or beliefs may affect the covariation judgment strategies they adopt. Whereas Experiment 5 showed no differences between older (50-75 yrs)and younger (16-36 yrs) adults in their conceptual understanding of inter-event correlations, Experiment 6 found that prior expectancies had rather different influences on the covariation judgments of older (50-70 yrs) and younger (16-36 yrs) adults. Finally, the third group of experiments (Experiments 7-9) considered how older and younger people put their health-related beliefs to the test of evidence. On the face of it, Experiment 7 suggested that older (51-79 yrs) and younger (18-32 yrs) adults showed similar understandings of what information is needed in evaluating hypotheses. However, the adoption of a computer-based process-tracing system (MOUSELAB 4.2, Johnson et al, 1989) in Experiments 8 and 9 suggested that there may be some subtle differences between older (18-36 yrs) and younger (50-70 yrs) adults in their selection and use of probabilistic data for hypothesis testing. The findings were discussed in relation to their specific implications for understanding the illness behaviour of older and younger adults, and their more general contribution to Behavioural Decision Research.