Dental caries is a common, global disease. Although caused directly by bacteria on the teeth, it is generally accepted that a large number of different factors are involved in the process. The interactions of these factors determine if the disease - and cavities - will occur or not. This complex background may be subject for several long and interesting discussions among scientists. Nevertheless, caries has to be explained in a simple, comprehensive way to people and patients, often under time pressure for the dental professionals.