Starting from the clinical observations that moderate haemophiliacs experienced only few bleeding episodes and rarely developed significant joint deterioration (haemophilic arthropathy), and the pioneer experience in Sweden, prophylaxis (i.e. the regular and long-term administration of clotting factor concentrate in order to prevent bleeding) has been practiced for more than forty years in severe haemophilia and is currently recommended as the first choice of treatment by the World Health Organisation and World Federation of Hemophilia and by many national medical/scientific organizations. Observational studies clearly established the superiority of prophylaxis over on-demand treatment in reducing the risk of arthropathy, also showing that starting prophylaxis earlier in life and after very few joint bleeds was associated with better joint outcomes, and led to the current definitions of primary (started before the age of 2 yrs and after no more than one joint bleed) and secondary prophylaxis. More recently, evidences from randomized trials, which were previously lacking in this setting, were also provided. This review summarizes available data from which current clinical practice of primary (and early secondary) prophylaxis in children with severe haemophilia was drawn. Open issues concerning optimal regimens and barriers to the implementation of prophylaxis are also discussed.