Epidemiology has been developing rapidly over the last half a century. It has evolved to a discipline which can serve as a basic science in both public health and clinical medicine, as they all involve information collected from human observational studies. As we are entering the information age in the 21“ century, all health care workers must be practically trained with the ability to judge and select valid information coming from studies of human population. The capability is particularly important as more and more health related information over flowing on the net and webpages. In this book my aim is to equip our readers with basic concepts and principles on how to conduct and critique scientific studies involving human population. Thus, the book is not only written for students majored in epidemiology as the first course and a conceptual review, but also for scholars, experts, and students who are not majored in epidemiology but need such concepts to abstract valuable information from or cooperate with others to conduct epidemiological studies.