Intracellular concentration describes the concentrations of a drug in a cell such as a bacterium, characterized by the penetration, distribution, degradation and efflux of drugs in individual cells. Antibiotics don’t distribute equally in all tissue compartments. β-lactam antibiotics, for example, hardly penetrate into intracellular compartments of the bacterial cell, while fluoroquinolones and macrolides may even accumulate intracellularly. Intracellular penetration is important for the therapy of intracellular pathogens such as Chlamydia, M. tuberculosis, Legionella, Salmonella typhi. The intracellular space has different intracellular compartments where bacteria and the drug may distribute differently.