Antibacterial agents are composed of a diverse group and many such agents have entered common usage through experience with little information on their mechanism of action. Study of the mechanism of action of an antimicrobial agent provides an insight into resistance mechanisms, toxicological problems and the design and development of new agents or combinations. The primary target of most antimicrobial agents (excluding antibiotics) is the cytoplasmic membrane and associated enzymes. Membrane-active agents can cause a change in the fluidity and/or permeability of the cytoplasmic membrane. Such changes can be determined by the leakage of cellular constituents such as potassium ions, nucleotides and their constituents and amino acids. The effect of an anti bacterial agent on the cytoplasmic membrane can also be determined by elucidating the effect of the antibacterial agent on the activity of membrane-bound enzymes and substrate uptake.