This handbook contains nuclear data needed by safeguards users for their work. It was initiated by an IAEA working group, and the contents were defined by the relies to a questionnaire sent to safeguards specialists. This is a preliminary edition of the handbook for distribution to safeguards and nuclear data experts for review. The present edition of the handbook contains the following basic nuclear data: actinides: nuclear decay data, thermal neutron cross sections and resonance integrals, prompt neutron data, delayed neutron data; fission products: nuclear decay data, thermal neutron capture cross sections and resonance integrals; fission product yields. Also included are appendices that summarize the data requested by safeguards users, and that present a number of questions to them and to data experts on the data contained in this preliminary issue and about additional data for possible inclusion in future editions and updates of the handbook.