Traditionally, conditioning regimens prior to hematopoietic cell transplantation (HCT) were designed to provide the highest dose of chemoradiotherapy tolerated by the patient with the hope of complete tumor eradication. This chapter addresses the conditioning options available for allo-HCT, and explores some of the controversies surrounding the selection of specific regimens. It presents eight case studies with multiple choice questions and answers for malignant hematological diseases.