Erosive tooth wear (ETW) is a common phenomenon in the general population. Furthermore, people with eating disorders or reflux, special diets, regular consumption of acidic foods and beverages, some medications or drugs, and occupational exposure to acids, including athletes, are at high risk for ETW. Differences in indices, sample sizes, study designs, in case of risk groups, small sample sizes and heterogeneous study groups, and lack of calibration of observers make it difficult to compare studies and estimate the true global prevalence. This chapter provides a narrative overview of data on ETW in the primary and permanent dentition of children, adolescents, and adults. There is an enormous variability in prevalence, ranging from 1 to 100% in the primary dentition (rough mean 30-50%), from <1 to 97% in the permanent dentition of children/adolescents (rough mean 20-40%), and from 2 to 100% in adults (rough mean 30-50%) over the world. Eating disorders are associated with increased prevalence, severity, and risk, although not all patients have pathological levels of ETW. There also seems to be a tendency for ETW to be more frequent and severe in case of reflux. Regarding exogenous causes, many studies, but not all, document a positive association between consumption of acidic drinks and ETW. Similar was found for use of drugs/medication and special diets. Although some associations between acid exposure and prevalence of ETW appear clear, the number of studies with large sample sizes is small. There is a lack of controlled prevalence studies, making it difficult to draw definitive conclusions for all (risk) groups.