ISCO-88, the revised edition of a major ILO publication, is an indispensable tool for describing and presenting information on occupations in a manner which makes international comparison possible. It can also serve as a model for developing or revising a national occupational classification. ISCO-88 has a hierarchical, four-layer structure, built on the basis of the type of work performed as reflected in the level, and in the specialization, of the skills needed to execute a job. A large number of countries have already started to develop their national occupational classifications according to ISCO-88 principles.