The book describes the different and exciting pathways which have been developed by pathogenic microbes to manage living inside host cells. It covers intracellular life styles of all relevant pathogenic but also symbiotic microorganisms with respect to the cell biology of the host-microbe interactions and the microbial adaptations for intracellular survival. It features intracellular trafficking pathways and characteristics of intracellular niches of individual microbes. The book also asks questions on the benefits for the microbe with regard to physiological needs and nutritional aspects such as auxotrophy, effects on genome sizes, and consequences for disease and host response/immunity (and the benefits for the host in the cases of symbionts). Additionally the book includes those pathogens that are medically less important but represent distinct intracellular niches, trafficking behaviours and virulence traits. The individual chapters also point out future challenges of research for the respective organism.