The topic of income inequality has become a new nexus of research among historians and social scientists recently. A certain amount of inequality is unavoidable in modern capitalist societies and economies – yet an increase in inequality may in time lead to severe challenges, even societal crises. European history alone has proved this time and again, with bloody revolutions and uprisings. Economic inequality, measured on the basis of incomes or wealth, decreased in Western countries in the first part of the 20th century. The phase of shrinking inequality was reversed after, first in the Anglo-Saxon countries and later in the Central Europe as well as in the Scandinavian countries. The disparities with data raise questions about how people should handle these types of issues – especially since these types of subtle methodological and analytical differences are not obvious to politicians and the lay public – and which sources people should prefer to use.