In addition to ISCO-08, three versions of ISCO were developed in the past. The first complete version of ISCO was adopted in 1957 by the Ninth International Conference of Labour Statisticians (ICLS), it is known as ISCO-58. ISCO-58 was superseded by ISCO-68, which was adopted by the Eleventh ICLS in 1966. The third version, ISCO-88, was adopted by the Fourteenth ICLS in 1987.

Distinctions between different occupational classes were introduced in population censuses undertaken in the early parts of the 19th century in several countries, but the identification of occupation and industry as different variables, each requiring its own classification, was only made towards the end of that century, as it became increasingly clear that the division of labour between firms and organizations in an industrial society was distinct from the division of labour between different jobs within the same firm.

The history of the development of ISCO has always been closely connected with the work of the International Conference of Labour Statisticians (ICLS) which meets under the auspices of the International Labour Organization (ILO).

The need for an international standard classification of occupations was, in fact, discussed at the first ICLS in 1923, although this conference did not propose a specific grouping of occupations. The first concrete step towards its establishment was the adoption, by the Seventh ICLS in 1949 of nine major groups termed the International Standard Classification of Occupations. The seventh ICLS proposed further development of this classification to more detailed level and identified basic principles to be followed in the collection and tabulation of occupational data, including that:

• the basis of any classification of occupations should be the trade, profession or type of work performed by an individual, irrespective of the branch of economic activity to which he or she is attached or of his or her status in employment;
• proprietors or owners who mainly perform the same work as that performed by employees in their own or in a similar enterprise should be allocated to the same group to which the employees are allocated.

In 1952, the ILO published the International Classification of Occupations for Migration and Employment Placement (ICOMEP), with descriptions of 1727 occupations based on national reference materials sourced principally from 13 countries, and organized within the framework of the nine major groups adopted by the Seventh ICLS.

In 1954, the Eighth ICLS approved a list of minor groups which was subsequently submitted to the governments and to a group of experts for comments.

In 1957, the Ninth ICLS adopted the first complete version of the International Standard Classification of Occupations known as ISCO-58 and comprising a classification structure of 10 major groups, 73 minor groups, 201 unit groups, 1345 occupations and an index of occupational titles. All groups at each level of the classification were given a unique code, title and description, drafted in consultation with the governments. It was published in 1958.

The Ninth ICLS also proposed a prompt revision of ISCO-58. In 1965, a working party examined proposals made on the basis of comments from about 80 countries and ten international organisations concerning the classification. This facilitated the development of a draft classification which was discussed, amended and adopted as ISCO-68 by the Eleventh ICLS in 1966. ISCO-68 consists of 8 major groups, 83 minor groups, 284 unit groups and 1506 occupational categories. It includes definitions for each of the 1881 groups describing the general functions of the occupations as well as the main tasks performed by the workers concerned. It includes an expanded alphabetical list of several thousand titles as well as a table of conversion from ISCO-58 to ISCO-68. The English and French versions were published in 1969 and the Spanish version in 1970.

The third edition of ISCO (ISCO-88) was adopted by the 14th ICLS in 1987 and approved by the ILO Governing Body in 1988. The major changes introduced were that:

• the underlying principles and concepts were made more explicit than in the previous versions;
• skill level and skill specialization were identified as the main similarity criteria for arranging occupations into groups; and
• the 10 major groups, 28 Sub-major groups, 116 minor groups and 390 unit groups in ISCO-88 provide much less detail than its predecessors, because experience had shown that it was very difficult to develop a comprehensive set of detailed categories that are applicable to all countries.

Early during the first decade of the 21st century, it became clear that there was a need to update ISCO-88. In December 2003 the 17th ICLS requested that the ILO complete work to update ISCO-88 by the end of 2007 so that the results could be taken into account in national preparations for the majority of population censuses to be undertaken in the 2010 round. The updated classification, ISCO-08 was adopted by a Meeting of Experts in Labour Statistics in December 2007.

Whilst it employs a similar conceptual model to that used for ISCO-88, and the 10 major groups were not changed in concept, ISCO-08 is slightly more detailed at its disaggregate levels and comprises 43 sub-major groups, 131 minor groups and 436 unit groups. Like its predecessor, ISCO-08 provides descriptions for all of the categories identified at each of the four levels of its structure, and can be extended by defining detailed occupations if and when required for specific national or regional purposes. ISCO-08 is now being revised.