Lead–acid battery (LAB) is the oldest type of battery in consumer use. Despite comparatively low performance in terms of energy density, this is still the dominant battery in terms of cumulative energy delivered in all applications. From a well-known car starter battery, to applications for lighting and interruptible power supplies, and to photovoltaic solar systems, lead–acid batteries have been the most commonly used battery type. Despite the emergence of several, more advanced battery systems, lead–acid batteries have persistently remained a universal choice for many applications. Their principle of operation, types, charge and discharge processes, components, and failure modes are explained in this chapter.