Batteries are important components in many technical systems that require decentralized storage of energy. Some examples are uninterruptible power supplies, stand-by batteries in power plants or telecommunications systems, remote photovoltaic systems, and automotive power systems. A growing demand is observed in battery application technology towards (1) battery monitoring tools that reliably indicate the state of a battery during operation, and (2) battery models for system simulation. Both of them require experimental characterization of the dynamic battery behavior.Impedance spectroscopy provides a unique tool for the analysis of the dynamic behavior of batteries. Compared to step-response methods, harmonic small-signal excitation allows direct measurement of system response in any working point. The precision of impedance measurements is not limited by the non-linearities or the long relaxation times, which are typical for electrochemical systems. Battery models can be obtained whose elements are closely related to the underlying physico-chemical processes. As a tool for indicating the state-of-charge (SOC) or the so-called state-of-health (SOH), impedance measurements might avoid the need for frequent capacity tests or additional sensors.