Publisher Summary This chapter focuses on the interactions between antimicrobial agents and microorganisms at a biochemical and molecular level. Antimicrobial agents are chemical substances that can either kill or inhibit the growth of micro-organisms. Such substances may be natural products or synthetic chemicals, and they are of undoubted value for the treatment of infection, and in well-defined circumstances they may be used for the prevention of infection. In order to be effective, antimicrobial agents must possess a selective action on micro-organisms without toxicity to the cells of the host. The sensitivity or susceptibility of micro-organisms to antimicrobial agents also varies. Gram-positive bacteria are usually more sensitive to antibiotics than are Gram-negative bacteria. Antibiotics that act on both Gram-positive and Gram-negative bacteria are termed broad-spectrum antibiotics and generally have a wider medical usage than narrow-spectrum antibiotics, which act on only a specific group of microorganisms or indeed only a single microbial species. Selective toxicity is achieved by exploiting differences between the structure and metabolism of microorganisms and host cells. Antimicrobial agents should have target-dependent selectivity, whereby the agent has an inhibitory action that is highly specific and directed towards sites and/or enzymes that are represented only in the target organism or cell.