Borehole acidization has two objectives: to remove drilling damage at the well face and to enhance formation permeability. Acid applications have been mainly on carbonates, granitic and metamorphic rocks in geothermal wells and on sandstones in oil and gas wells. In geothermal wells, acidizations have been especially useful in removing accumulated scale deposits. Hydrochloric acid is the most commonly used as it has a high dissolving power, a lower cost and is relatively easy to handle. It reacts easily with carbonates but not with silicates in sandstones for which a mixture of hydrofluoric and hydrochloric acid is used. There are no known water well acidizations with hydrofluoric acid. Acidization of limestone water wells with hydrochloric acid has been generally successful in naturally fractured rock with productivity improvements of two or more times the original yield. Second and third acidizations can enhance yields further and are usually economically justifiable. Water well acidizations may benefit from higher injection rates than is currently practised. Acid fracturing is widely used in the oil and gas industry. In water wells it may prove useful in hard crystalline limestones, but not in soft low strength carbonates, such as UK Chalk.