How accurate are statements about smoking habits? This book presents the results of a comprehensive review in which the literature on the subject is newly interpreted. It is shown that smokers are misclassified as non-smokers in epidemiological studies often enough to explain the increased lung cancer risk seen in self-reported non-smokers in relation to their spouse's smoking habits. This study overturns the commonly held view that increased risk is a consequence of exposure to environmental tobacco smoke and highlights the difficulty of making valid inferences from epidemiological data. No-one should draw conclusions about passive smoking before reading this book!