The issue of lexical modernisation in Arabic, particularly in the area of science and technology, has been raised for a long time with concentration mainly on the methods of lexical expansion adopted by language planning agencies (language academies and other language planning organisations) or individuals concerned in this process such as dictionary compilers and translators. The problem of lexical modernisation does not lie solely in finding ways of word creation and inventing lists and glossaries of new words. More important is whether these words are being adopted and used by the language community or not, and if not, why and what are the possible reasons behind accepting or rejecting certain terms. This study is an attempt to answer these questions making use of the vocabulary produced and/or co-ordinated by Arabic language planning agencies or individuals, a list of which is included in the bibliography. The study is divided into seven chapters commencing with a discussion of some major aspects of language planning in general and Arabic language planning in particular. In attempting to answer the above questions, it is believed that there are two kinds of factors that might affect the acceptability of newly introduced vocabulary: (1) linguistic factors, which include the features of the term itself i.e. word length, ease of pronunciation, transparency, and the factor of diglossia, and (2) extralinguistic factors, those to do with the planners such as lack of authority and financial resources to distribute, or the speakers including lack of motivation, illiteracy or other factors such as contact with foreign languages with considerable prestige. The researcher relied mainly on some questionnaires conducted in the University of Tripoli in Libya, and among specialised groups of speakers, in addition to consulting speakers with intensive contact with technical vocabulary. For the problem of terminology disunification, a survey of some lexicons and glossaries was conducted.