In order to avoid duplication between standardization at international and European levels, for the benefit of contributors and users of standards as well as to increase the efficiency of standardization at European and international level, CEN and CENELEC have signed agreements with their respective international counterparts the International Standardization Organization (ISO) and the International Electrotechnical Commission (IEC), setting out the rules governing co-operation.
The Vienna Agreement signed between CEN and ISO in 1991 recognizes the primacy of international standards and aims at standards to be recognized simultaneously at international and European level by means of improved exchange of information and mutual representation at meetings. Either CEN or ISO shall take the lead in drafting a new standard and documents related to them shall be presented for simultaneous approval by both. This way, ISO members can influence content of CEN standard and vice versa. Approximately 31% of CEN standards are developed under the Vienna agreement.

Nevertheless the Vienna Agreement allows CEN or ISO to conduct standardization activities on the same subject, if deemed necessary. The “Joint ISO-CEN Coordinating Group of the Technical Boards” has an important strategic function of monitoring application of the Vienna Agreement and of advising the higher CEN Technical Board and ISO Technical Management Board on all issues relating to the Vienna Agreement, i.e. including on the need for revisions. A revised version (Version 3.3) of the Vienna Agreement was released in September 2001, in which the agreement itself was reduced to the essential principles of co-operation between ISO and CEN. Altogether, the current version gives priority to international standardization, and lends greater importance to ISO leadership than did the previous versions. EN ISO standards, for example, may now be revised only under ISO leadership, regardless of their origin.
In 1996 CENELEC and IEC signed the Dresden Agreement in order to create the necessary framework for an intensive consensus-finding process between European and international standards development activity in the electrical sector.

In contrast to CEN, CENELEC has undertaken to have all new standardization projects conducted by IEC at international level, if possible. Voting on international standards in IEC is always conducted in parallel at CENELEC; international standardization projects are therefore automatically also European standardization projects. Only where IEC is not interested in a standardization project may the work be conducted solely at European level. In this case, CENELEC must however keep IEC informed of the activity and allow IEC to comment during the public enquiry stage.

The close intermeshing of European and international standardization activities through the Vienna and Dresden Agreements has led to some 31% and 76% of all European standards adopted by CEN and CENELEC now being technically equivalent or identical to ISO and IEC standards respectively. This high proportion of uniform standards facilitates implementation of the WTO Agreement on Technical Barriers to Trade (WTO-TBT) on the global market.