We discuss the following fundamental problems of early Solar System history.
(i) Fine chronology, using long–lived U–Pb as well as the different extinct radionuclides.
(ii) The nature of the solid material from which the planets were made.
(iii) The immediate prehistory of the formation of the Solar System including the question of a possible supernova trigger.
(iv) The origin of anomalies in oxygen isotopes.
In each case, an effort has been made to distinguish what we know from what we need to learn.