Impurities in fusion plasmas have two detrimental effects that may render the achievement of ignition difficult: dilution of the reacting plasma reduces the produced fusion power and radiation losses prevent efficient heating up to ignition temperatures. In tokamaks, impurities are released from the containing vessel walls by plasma--wall interaction. They are ionized when entering the plasma and transported into the central plasma by transport processes that are not understood. The present impurity situation in JET is reviewed in this paper: the main contaminants are carbon, oxygen and nickel, as diagnosed by spectroscopic methods. The concentrations depend on the wall conditioning methods, on the plasma parameters (for example, current and density) and on the power of the applied heating methods. The dilution of hydrogen varies between 20 and 50% and the total radiation losses range between 30 and 100%. The central radiation losses are usually small compared with the local heating power. Application of radio-frequency heating in JET does not lead to excessive contamination, in contrast with other experiments.