This book provides information on the molecular interactions between host cell organelles and pathogens, which have developed strategies to survive within infected cells. Chapters are grouped into five sections: I. Endocytosis and phagocytosis. Collectively, the chapters of this section review basic knowledge regarding intracellular organelles are involved in membrane interactions with pathogen-containing vacuoles. II. Professional and non-professional phagocytes . Here the authors describe the major differences between the two host cell types, which can be infected by microorganisms. III. Maturation pathways of bacteria-containing vacuoles . Molecular interactions between vacuoles and intracellular organelles leading to the search of the Holy Grail, the replication niche, are described. IV. Host response. Host cells are able to react against intruders and eventually mount host responses. In these chapters the various types of host response mechanisms against intracellular intruders are reviewed. V. Co-evolution. In these final chapters, the question is addressed of whether knowledge of bacteria-host cell interactions will be acquired fast enough to find the necessary tools for controlling microorganism development.