Surprisingly what separates us from the open environment all around us sometimes is a single layer of epithelial cells. It is at these seemingly fragile sites that most pathogens, including HIV, influenza, emerging and biodefense agents, gain access to our inside milieu. While there are major similarities between the cells and the immune responses generated at the mucosal membranes of the gastrointestinal and respiratory tracts together with the genitourinary tract, there are also important differences. Knowledge of these differences and similarities is required in order to understand the interactions between us, as the host, and the pathogens that attack through each tract, and how our immune system reacts to each of them. Whether we want to devise rational prophylactic or therapeutic vaccines or treatments to either prevent or treat mucosal infections we must acquire such knowledge. This is the rationale behind putting this book together. This book will provide the readers in the areas of vaccinology, virology, bacteriology, epidemiology, immunology and mucosal immunology within academia (undergraduate, graduate, post doctoral fellows and professors), as well as preclinical and clinical scientists in vaccine and drug industries a thorough appreciation of the mucosal immune system and its importance in protecting humans against mucosal pathogens.