Pathogenic micro-organisms receive much attention due to their potentially harmful effects on human, animal or plant health.Over the past few decades, this attention has grown due to the emergence of new (and known) infectious diseases inducing local epidemics as well as worldwide pandemics.Along with the research and diagnosis of those etiological agents, (bio)safety concerns have highlighted the biological risks associated with their deliberate use in laboratories, animal facilities and production plants, and their transboundary movements (import and export). This has led to their categorisation into risk groups and the elaboration of classification lists.Current international classification systems rely on criteria defined by the World Health Organisation, which cover the severity of the disease the micro-organism might cause, its ability to spread and the availability of prophylaxis or efficient treatment. Animal pathogens are classified according to the definitions of the World Organisation for Animal Health, which also consider economic aspects of disease.Evolution of scientific knowledge will demand regular updating of classification lists. This paper describes the Belgian risk classification system and the methodology that was used for its peer-reviewed revision (with a focus on animal pathogens).