This book is an introductory text for those people who need to know more about the scientific principles behind the study of the causes of disease occurence. As most people spend a large part of their lives at work, and as some people may be at risk of being exposed to harmful substances at high levels, workers protection should have a high priority. Studies in the workplace can help to identify causes of occupational diseases, and the outcome of such investigations should be applied when the effects of protective action are evaluated. In searching for occupational causes of diseases, epidemiological principles should be used, even at the factory level. Applying strict logical principles to the collection of data concerning worker's experience with ill-health ensures that a useful insight is gained into the causes of disease, and that data are obtained in a format suitable for subsequent larger-scale studies.