PLASYD is an ICL internal language which has been used in the production of several of the company's major software products. The most significant example is the optimising FORTRAN compiler, XFEW, and its derivatives. The Atlas Computer Laboratory has permission to use PLASYD on the understanding that the product is unsupported and not necessarily bug-free. This manual is mainly a rewrite of ICL internal publications with modifications indicating how PLASYD will be available on the Laboratory's 1906A. We would like to thank ICL for their co-operation in making the necessary internal publications available to us and permission to incorporate parts of them in this manual. In particular, we would like to dedicate this manual to J K Buckle, of ICL, the designer of the language.