There are mandatory standards which are enforced through EU regulations or directives. They represent about 20% of standards produced.  Products being put on the EU market must undergo conformity assessment by an EU notified body to demonstrate that they comply with the harmonised standard. Examples include medical devices almost all of which are required to have a CE Mark on the product. 

There are also voluntary standards which represent the bulk of current standards.  Broadly speaking, there are 4 types of standards:

• International standards
• European standards
• Harmonised European standards
• National standards

Harmonised standards are produced in response to a mandate from the European Commission to the European standards organisations, the Comité Européen de Normalisation (CEN), the European Committee for Electrotechnical Standardisation (CENELEC) and the European Telecommunications Standards Institute (ETSI).  When adopted, they are listed in the Official Journal of the European Union. Products within the scope of the relevant Directive produced in accordance with harmonised standards will enjoy a presumption of conformity with the relevant essential requirements (ERs).

European standards are voluntary recognised standards issued by one of the three EU standardization bodies (the Comité Européen de Normalisation [CEN], the European Committee for Electrotechnical Standardisation [CENELEC] or the European Telecommunications Standards Institute [ETSI]) and are developed in conjunction with interested parties such as manufacturers etc.  They impose no obligation but indicate a recognised standard to which certain products should conform.