This paper reports on a new project, Technical Language and Semantic Shift in Middle English, which aims to address outstanding questions about semantic shift, lexical and/or semantic obsolescence and replacement. In order to consider the pressures exerted by lexical borrowing, the project takes Middle English as its initial sample. The project investigates how changes in semantics and lexicon differ at the various levels of the lexical hierarchy, with a particular interest in technical language. The data set consists of the Middle English terms collected for the Bilingual Thesaurus of Everyday Life in Medieval England, terms for building, domestic activities, farming, food preparation, manufacture, trade, and travel by water; with the addition of vocabulary from the domains of hunting and medicine all arranged into a semantic hierarchy. We also include three case studies illustrating how we used lexicographic information in the construction of the semantic hierarchy; how data are interpreted in relation to change within a particular semantic field; and how obsolescence can be understood through drawing a distinction between obsolete lexemes and obsolete senses.