Antibodies enable relatively simple, rapid, and reproducible studies to be performed on protein expression in both normal and neoplastic cells. The availability of suitable reagents for routine diagnostic use has revolutionized the study of leukemia and lymphoma pathology. The use of antibodies specific for anaplastic lymphoma kinase (ALK) protein has permitted the identification of the tumor entity ALK-positive from the poorly defined morphological category of anaplastic large cell lymphoma. Antibody techniques have also provided major new insights into the biology of these tumors. This chapter describes the use of antibodies in both immunolabeling (immunocytochemical and immunofluorescence) procedures and in biochemical procedures in the study of ALK-positive ALCL.