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Tetracyclines block amino-acyl tRNA attachment to the 30S ribosomal subunit, preventing bacterial protein synthesis; their action is bacteriostatic. Their spectrum includes Gram-positive and Gram-negative bacteria, atypical organisms, rickettsiae, spirochetes, Brucella, Vibrio, and Plasmodium, though activity can be reduced in high-inoculum infections or acidic pus. The lesson distinguishes core agents and tigecycline, and relates selected agents to clinical applications. It also considers factors that affect oral absorption and excretion, along with characteristic adverse effects, precautions, interactions, and resistance mechanisms. These principles help explain why tetracyclines have useful applications as well as important restrictions.