According to the original concept of this Handbuch series, the Sauropterygia were supposed to be treated in a single volume, Part 12. However, the increase of new fossil material and intensive research has led to a wealth of published data during the last decades. As a consequence, the Sauropterygia as a whole had to be split into two parts for the Handbuch purposes, resulting in Part 12A dealing with the Triassic Nothosauria, and Part 12B dealing with the post-Triassic Plesiosauria. Now, the organization of the two volumes will follow the new concept of Rieppel, understanding the Sauropterygia as a clade divided into stem-group Sauropterygia from the Triassic, comprising Placodontia and Eosauropterygia (sensu Rieppel) combined in this volume, Part 12A, Sauropterygia I. Placodontia, Pachypleurosauria, Nothosauroidea, Pistosauroidea, and into crown-group Sauropterygia from the Jurassic and Cretaceous, Part 12B, in preparation, comprising plesiosaurs, pliosaurs and elasmosaurs. In addition, inclusion of the Placodontia in this volume has the advantage of presenting an updated revision of this group as a whole not only in the context of this Handbuch series, but also in general.