This is a study based on observation rather than mathematical theory in which an attempt is made to discover facts and relationships that give evidence about the course of solar evolution. The author's principle conclusions are that the earth and other territorial planets were formed at much lower temperatures than has generally been supposed, and that a more uniform distribution of iron throughout the silicate phases of the Earth existed in the past than exist now while the iron core of the Earth has been formed at least partly during geological time.For convenience of discussion the author divides the course of planetary evolution into five stages: stage 1 is assumed to start with an interstellar dust cloud, such as the globes of Bok and Reill, stage 2 he calls the preprotoplanet and protoplanet phase, when temperatures had risen to their present levels, stage 3 is the "high temperature stage", and stage 4 is the second low temperature stage, in his final stage the author outlines possible origins for the atmospheres of the earth, Venus and Mars.It may at first surprise that a physical chemist should write a book on the origin and development of the planets. But as the physical problems are many and involved and as the evidence in regard to them is very detailed, the physical chemist can obviously make valuable contributions to this subject. Mr Urey has discussed his views with astronomers, physicists , geophysicists and many other experts and frequently made use of their critical appreciation.