Compositional data are a special case in the field of statistics. They are non-negative data with the distinctive property that their values sum to a constant, usually 1 or 100%. They are measures of parts (or components) of some total, where the total is usually of no interest. Their special character is due to the fact that their values depend on the particular choice of parts constituting the total amount. In this chapter compositional data are introduced and the use of ratios, preferably logarithmically transformed, is justified.