An action potential (or nerve impulse) is a transient alteration of the transmembrane voltage (or membrane potential) across an excitable membrane in an excitable cell (such as a neuron or myocyte) generated by the activity of voltage-gated ion channels embedded in the membrane. The best known action potentials are pulse-like waves of voltage that travel along the axons of neurons. This book reviews research on action potential including an overview of the role of specialised axonal excitability techniques in understanding the consequences of abnormal membrane excitability; the physiological implications of action potential in characean cells; and action potential production and the ion channel dependent process and others.