Publisher Summary This chapter shows that host–invader interplay is a complex phenomenon that deals with different stages of phagocytosis or of the digestion process according to the pathogen. The best understood mechanism of pathogen escape is that used by bacteria that is they are able to grow outside cells. They succeed in avoiding adhesion and phagocytosis because of their peculiar surface properties. The study of microbe phagocytosis showed that, in most cases, microbes are killed and degraded either by blood phagocytes or by monocytes or macrophages. Some microbes, however, can either avoid phagocytosis and so invade the tissues or survive and multiply inside macrophages. The ability of intracellular parasites to escape from the microbicidal activity of professional phagocytes confers to them pathogenic properties that in many cases create severe problems of public health. Pathogen resistance is multifactorial and only exceptionally the property of a single determinant. Microbes must first inhibit or resist the oxidative burst and then neutralize the cationic proteins and lysosomal enzymes either by preventing lysosome fusion or by resisting lysosomal content.