Non-carious tooth tissue loss due to abrasion, attrition, abfraction and erosion has become a significant problem, occurring in up to 80 percent of children and in up to 43 percent of adults. Dental erosion is now recognised as an important cause of tooth tissue loss in both children and adults. It is caused by the presence of intrinsic or extrinsic acid of non-bacterial origin in the mouth. Intrinsic sources of acid include vomiting, regurgitation, gastro-oesophageal reflux or rumination. Extrinsic sources of acid are most commonly dietary acids. Medications, a patient's lifestyle choices and environment can also increase the risk of dental erosion. In this article we identify the prevalence of dental erosion and its main causative factors, and in Part II we will discuss the management of dental erosion.