Cambridge, CUL, MS Ee.1.13 contains a Middle English version of the 'Liber de Simplici Medicina' ('Circa Instans'), a treatise on 'materia medica' attributed to the twelfth-century physician Matthaeus Platearius. The relevance and popularity of this Latin work composed at the renowned medical School of Salerno extended over time and space. Its encyclopaedic presentation of 'simples' of vegetable, animal, and mineral nature - with allegedly curative virtues - was the core of botanical works until the sixteenth century, and has survived, in various forms - treatise, remedy book, auxiliary tables - in a substantial number of manuscripts in Latin and the vernaculars. This is the first edition of the complete version of the 'CI' in Middle English. The introduction, commentary, and glossary not only provide a context for reading the text, but also contribute to an understanding of the emergence and development of a language and culture of medicine in medieval England. The glossary is an extensive compilation of the anatomical, medical, and botanical terminology used.