The problem of anaplastic large cell lymphoma (ALCL) is extensively reviewed by depicting the clinical, pathological and biological characteristics of the four main varieties of ALCL: common, Hodgkin's like/Hodgkin-related, lympho-histiocytic, and giant-cell rich. Special emphasis is given to the differential diagnosis between ALCL Hodgkin like and Hodgkin's disease in the light of possible therapeutical differences.