This volume represents one of the first full-length studies carried out on material from the International Corpus of Learner English (ICLE), supplemented by data from younger learners and native speakers. It addresses three main goals: a) the implementation of a developmental corpus methodology. The study explores four corpora of argumentative writing, two sampled from advanced learners of different ages and two from corresponding native speakers of English. This way, the respective linguistic maturation in native and non-native writing can be traced with more explanatory power than could be yielded by a mere learner / native speaker contrast. b) a functional account of adjective intensification in present-day written English. Intensification is a singularly dynamic and innovative lexico-grammatical class. Despite their obvious limitations, small, text-type controlled corpora, such as the ones used here, make it feasible to examine this whole functional paradigm and identify the conceptual mechanisms of its continual innovation and semantic change. c) the exploration of native vs. non-native usage and the notion of idiomaticity. The main differences between native English usage and that of advanced learners rest not so much on grammatical structure, but on the rather elusive quality of 'idiomaticity'. In the limited domain of intensification, this notion is explored both qualitatively and arithmetically, with the aim of learning more about what it takes to use English idiomatically.