This thesis is an investigation of Middle English insanity language. It analyses change in the Middle English lexical field of INSANITY, the semantic structure of lexemes wod and mad, and compares INSANITY conceptual metaphors in Middle English and present-day English. The INSANITY lexical field is an ideal one to study language change, due to socio-cultural changes since the Middle Ages such as advances in medical knowledge, the development of the field of psychiatry and legal changes protecting people with a mental illness from discrimination. The general theoretical aims were to examine a) change in conceptual metaphor, and b) semantic and lexical change with a particular focus on the decline in use of adjective wod. The theoretical frameworks are cognitive linguistics, prototype theory, and conceptual metaphor theory, and the data is derived from Middle English corpora and other sources. The INSANITY database I created for this study consisted of 1307 instances of mad, wod and near-synonyms in context. The main results can be divided into three groups. Firstly, the lexical field study demonstrates that various intra-linguistic and socio-cultural phenomena effect lexical change. Using case studies amongst others of the decline of wod in the Wycliffite Bible and of Caxtonâs translations from French, and a systematic variation across genre, I argue that the important factors are i) the arrival of new medical loanwords such as frensy, lunatic and malencolie; ii) the early re-emergence of the vernacular in medical texts starting in the twelfth century, and the development of a new medical register; iii) the so-called medieval âinward turnâ; iv) changes in the neighbouring lexical field of ANGER. Secondly, the semasiological study of wod and mad shows that the meanings of these two lexemes are structured and change in line with the central tenets of prototype theory, i.e. as described for diachronic prototype semantics by Geeraerts (1997). The path of madâs semantic development does not parallel that of wod after the thirteenth century. Madâs senses do not have the emphasis on wildness and fury that the senses of wod do. A particularly interesting finding is the semantic change from a sub-sense of adverb mad and adjective mad, âunrestrainedâ, leading in present-day English to a new delexicalised and grammaticalised sense of mad, where its use as an intensifier enhances scalar quantity and quality. Thirdly, the conceptual metaphor study demonstrates that predominantly the same conceptual metaphors are seen in both Middle English and present-day English, with some exceptions such as the concept of insanity being related to moral decline, as evidenced in the dearth of FALLING metaphors for insanity in present-day English. Conceptual metaphors such as INSANITY IS ANOTHER PLACE are evidenced in present-day English expressions such as out of her senses, or not in my right mind. In 1422, Thomas Hoccleve could write of a dysseveraunce between himself and his wit, or about his wyld infirmitie, which threw him owt of my selfe, illustrating the same underlying concepts. Other INSANITY conceptual metaphors which remain unchanged are GOING ASTRAY, LACK OF ORDER, LACK OF WHOLENESS, DARKNESS, FORCE, PRISON and BURDEN. Because of its unique approach in combining onomasiological and semasiological approaches with a conceptual metaphor study, this study reveals not only specific patterns of change, but differences in the rate of change on the lexical and conceptual levels. Lexical change driven by the need to be expressive, and reflecting socio-cultural changes such as changes in medical knowledge, can be seen to happen rapidly over the Middle English period. However, underlying conceptual change is barely discernible even over a much longer period of time from Middle English to present-day English. This research is significant because it provides a basis for future analysis of insanity language in other periods and contexts. It also contributes to the study of semantic change in general, highlighting the insights that can be gained by combining different types of data-driven analyses.