The book takes up the subject of dictionary use from the perspective of advanced learners. The study aims to explore the effects of the use of a monolingual learner's dictionary on students' performance in a complex comprehension task, i.e. the task of interpreting fragments with modified idioms, which often disrupt the fluent reading process. The theoretical part summarises the results of lexicographic research in the field of receptive dictionary use and discusses its methodological aspects. Moreover, it introduces relevant elements of the reading theory and analyses the nature of idiomatic expressions, their transformations in particular, from a psycholinguistic point of view. Finally, problems connected with the presentation of idioms in monolingual learner's dictionaries are highlighted. The major, empirical part reports on an experiment, whose aim was to find connections between the consultation process, the way of presenting lexicographic information on idioms, and comprehension scores. The results reveal a mildly positive influence of the monolingual learner's dictionary on reading comprehension performance. Among the reasons for underachievement were misinterpretation of entries and insufficient processing of dictionary information in context.