||Despite the overwhelming evidence that smoking cessation reduces the risk for several chronic diseases, information on the magnitude of these public health benefits is scarce. It has furthermore been suggested that smoking cessation also improves health-related quality of life, but this has not been conclusively shown. The aims of this thesis were to quantify future public health benefits of smoking cessation and to give more insight into the impact of smoking cessation on health-related quality of life. First, the associations between smoking or smoking cessation and other risk factors for tobacco-related diseases were studied. Using ecological data of the Seven Countries Study, comprising middle-aged men in 16 cohorts in Europe, the United States and Japan, 25-year lung cancer mortality rates among smokers were higher in cohorts with high fat intake than in those with low fat intake, especially for saturated fatty acid. We were not able to conclude whether the effects of smoking and saturated fatty acid intake on lung cancer mortality were independent or that effect modification played a role. In the Doetinchem cohort study among 20-59 year old Dutch men and women, smoking cessation was associated with higher HDL cholesterol levels, which may contribute to the favourable effect of smoking cessation on cardiovascular diseases (CVD). Unfavourable postcessation weight gain partly explained the increase in total cholesterol level and blood pressure after smoking cessation. The effect of smoking cessation on future morbidity and mortality was estimated with computer simulation modelling. We first estimated that a reduction in smoking prevalence to 20% in 2015 in each member state of the European Union (EU) (WHO's `Health for All` target) would lead to a reduction in pancreatic cancer patients up to 2015 of 29,500 males and 9,500 females in the EU. Secondly, we observed that this target was expected to reduce the number of total deaths up to 2015 with 2.5% (around 1.1 million deaths) among men and 0.8% (almost 350,000 deaths) among women in the EU. These reductions in mortality were about 30%-50% of those achieved if all smokers would quit instantly. Moreover, applying the Framingham risk function to 40-74 year old Dutch smoking men and women with a high risk for coronary heart disease (CHD), we estimated that smoking cessation, although less or comparably effective in primary prevention of CHD, would lead to a 2-2.5 years larger gain in healthy life expectancy than cholesterol lowering medication. Finally, Dutch male and female ex-smokers reported higher health-related quality of life scores than current smokers, especially for mental health dimensions, in a cross-sectional study. Generally, the higher the amount of smoking, the larger were quality of life differences between ex- and current smokers. In conclusion, results in this thesis are one of the first to quantify the effects of smoking cessation on future morbidity and mortality. This information is useful to policy makers, in order to adjust future health care to smoking behaviour changes. Furthermore, this thesis provides evidence that smoking cessation leads to changes in other classical CVD risk factors. This implies that the favourable effect of smoking cessation on CVD risk may partly be mediated by postcessation changes in other CVD risk factors. Finally, our results suggest that smoking cessation also may improve health-related quality of life, especially mental health, but this finding needs to be confirmed in cohort studies.