This is a clinical practice guideline for adults and children with hematologic malignancies, solid tumours, or hypoproliferative thrombocytopenia. The guideline examines when and how clinicians should use platelet transfusion to prevent or manage bleeding in these patients. Recommendations are made on topics such as the preparation of platelet products, the prevention of Rhesus (Rh) alloimmunization, and platelet transfusion thresholds for patients with different conditions.