Blood transfusion services have the responsibility to collect blood only from donors who are at low risk for any infection that could be transmitted through transfusion and who are unlikely to jeopardize their own health by blood donation. A rigorous process to assess the suitability of prospective donors is therefore essential to protect the safety and sufficiency of the blood supply, and safeguard the health of recipients of transfusion and blood donors themselves, while ensuring that suitable donors are not deferred unnecessarily. These WHO guidelines on assessing donor suitability for blood donation have been developed to assist blood transfusion services in countries that are establishing or strengthening national systems for the selection of blood donors. They are designed for use by policy makers in national blood programs in ministries of health, national advisory bodies such as national blood commissions or councils, and blood transfusion services. The guidelines are presented in two parts. Part 1 addresses the requirements for an effective national system for blood donor selection and policy. Part 2 provides guidance on specific criteria for blood donor selection in relation to general donor assessment, donor medical history and risk assessment for transfusion-transmissible infections and technical recommendations on donor selection criteria.