Severity classification systems (ie pain scales, categories of invasiveness, degrees of severity etc) are used to classify the adverse effects experienced by animals used for scientific purposes. Currently, eleven countries use severity classification systems. These systems have developed
in various ways, depending on each country's process for overseeing the use of animals in science, as well as the particular aspects emphasised by those individuals who have championed their implementation. Severity classification serves four main purposes: as a tool to assist animal ethics
committees in ethical review; education of animal users about concepts for humane animal experimentation; provision of data to inform the public about scientific animal use; and provision of data to inform national policies. At a time when the newly accepted European Union Directive will make
the reporting of severity data mandatory, we review the characteristics of international severity classification systems and how they have evolved; analyse the effectiveness of some systems; and identify emerging challenges for severity classification.