The pharmacokinetics of tetracyclines and glycylcyclines are described in three groups. Group 1, the oldest group, represented by tetracycline, oxytetracycline, chlortetracycline, demeclocycline, lymecycline, methacycline and rolitetracycline is characterized by poor absorption after food. Group 2, represented by doxycycline and minocycline, is more reliably absorbed orally, while group 3, represented by the glycylcycline tigecycline, is injectable only, with an improved antibacterial spectrum compared with the tetracyclines. Though incompletely understood, the pharmacodynamic properties of the tetracyclines and glycylcyclines are summarized.