The main function of language is to convey meaning. The question of why language is structured the way it is, Heine here argues, has therefore to be answered first of all with reference to this function. Linguistic explanations in terms of other exponents of language structure, e.g. of syntax, are likely to highlight peripheral or epi-phenomenal rather than central characteristics of language structure. This book uses basic findings on grammaticalization processes to describe the role of cognitive forces in shaping grammar. It provides students with an introductory treatment of a field of linguistics that has developed recently and is rapidly expanding.