This book is about mechanisms of intracellular parasitism of Legionella, Listeria and Mycobacterium. The presented data illustrate the hypothesis that one of the ways by which intracellularly multiplying bacteria influence the bactericidal response of phagocytes is by altering signaling cascades in eukaryotes. To achieve this goal bacteria are capable to produce enzymes and other biologically active compounds which misdirect signaling pathways for the benefit of the parasites. The book is composed of two main parts. The first provides general information on signaling cascades in eukaryotic cells. Analysis of Legionella, Listeria and Mycobacterium products possessing regulatory activity toward metabolism of host cells, essentially by affecting signal transduction in eukaryotes, is performed in the second section.