Histiocyte disorders are characterised by tissue infiltration with cells of monocyte/macrophage lineage, with two disorders, Langerhans' cell histiocytosis (LCH) and haemophagocytic lymphohistiocytosis (HLH) accounting for the overwhelming majority of cases in childhood and, apart from monocyte variants of acute myeloid leukaemia, histiocytic malignancy is very rare. Although both LCH and HLH are considered reactive disorders, the prognosis of these conditions differs greatly, LCH is usually self limiting, with a mortality of 10%, but HLH is usually fatal, with a mortality of over 80% in the absence of bone marrow transplantation. Increased levels of cytokines have been demonstrated in these disorders, and may be responsible for many of the clinical features: it is unclear whether histiocytes themselves, or other immune cells, particularly T lymphocytes, are the abnormal cell population. Due to the rarity of histiocyte disorders, collaborative studies are essential to improve understanding and advance treatment.