Since the discovery of Legionella pneumophila in 1976, its significance increased rapidly in terms of public health and it became an important pathogen. With the discovery of a new kind of species, the life cycle of Legionella pneumophila has been investigated and interestingly, it was found that humans are an accidental host of the life cycle of these bacteria. L. pneumophila constitute an important part of community originated cases of atypical pneumonia and travel associated diseases, so it is important that there exists strict control in man-made habitats. Moreover, understanding the biology of L. pneumophila is critical for the development of more effective combat methods. During the preparation of this book, a large number of valuable works were examined and all aspects of L. pneumophila were introduced. The different features of this interesting microbe's journey from the existence in the environment to the diseases caused in humans were discussed and the prevention methods also have been mentioned. This book is a good start for researchers who want to have a first overlook at this subject.