This thesis explores the variability of verbal idioms in English and Croatian in order to determine the constraints which govern it as well as potential differences between conventional and nonconventional uses of idioms. Moreover, the thesis examines which components make up the idiom core, defined as the fundamental part of an idiomatic configuration guaranteeing its recognizability across lexical and grammatical variants. The theoretical framework of cognitive linguistics is used. The research has been conducted using the enTenTen13 corpus for English (19.6 billion words) and the hrWaC corpus for Croatian (1.2 billion words, version 2.2), searched using the Sketch Engine. A total of 380 English and 385 Croatian verbal idioms were analysed. The idioms were isolated by identifying the most characteristic noun collocates of 500 most frequent verbs, and isolating those constructions which were figurative in meaning. The resulting verbal idioms were then used as the starting point in a number of searches in the two corpora so as to obtain their variant forms, and to determine which of those are variations (conventionalized through frequent use) and which are modifications (created ad hoc for a particular communicative purpose in a particular context). The thesis is organized into an introduction followed by a theoretical and empirical part. The theoretical part starts with Chapter 2, which provides a brief overview of the existing definitions of idioms, discusses basic characteristics of idioms, and deals with the definitional scope of the category. Chapter 3 compares different theoretical positions on the relationship between the meaning of idiomatic components and the meaning of the expression as a whole, and discusses the contribution of conceptual and structural factors to idiomatic meaning. Chapter 4 addresses sentence patterns that appear in idioms from the point of view of cognitive grammar. It provides a description of sentence parts that make up idioms, their semantic roles, types of events that grammatical constructions may reflect, and the participants that take part in these events. Chapter 5 is dedicated to several issues. Firstly, it compares how idiom variability has been described theoretically, and discusses factors that contribute to the potential change of an idiom’s lexico-grammatical structure. Next, it discusses the boundaries between variation and synonymy, and then addresses the issue of canonical form as well as the relationship between variations and modification. The chapter concludes with an overview of traditional and corpus approaches to modifications and the existing research on this issue in English and Croatian. The empirical part of the thesis starts with Chapter 6 which lists the aims, hypotheses and the methodology, including the procedure and a detailed overview of the corpus searches used to obtain variations and modifications in the two corpora. Chapter 7 concentrates on conventional and nonconventional uses of verbal idioms. Three types of conventional changes have been identified: (1) replacement of the verbal component by other verbs, (2) morphological changes (nominalization of verbal idioms and using whole idioms or their parts as modifiers in a noun phrase), and (3) grammatical changes (word order, passive and the middle construction, transitive, intransitive and reflexive constructions, ellipsis of the lexical verb, negation and a subsection on changes in the grammatical structure accompanied by lexical changes). The section on modifications examines their characteristics and compares their functions in English and Croatian, focusing in particular on the extent to which the lexical composition and structure of an idiom may be changed in nonconventional use. Chapter 8 contains a general discussion of the results and their significance in relation to the research questions and hypotheses. The thesis ends with a conclusion. Three groups of results were identified. Firstly, meaning and grammatical structure go hand in hand in restricting the variability of verbal idioms in both English and Croatian (regardless of whether these changes are lexical or grammatical), and these restrictions work on a local and global level. Secondly, similar restrictions apply in both variations and modifications. Although modifications are impossible to predict, they exhibit conventionality on the conceptualgrammatical level, and are distinguished from variations by frequency of use. Thirdly, salient components are combined into the verbal idiom core in English and Croatian based on general patterns, and the idiom core consists of a conceptual and a structural part. The methodological contribution of this thesis consists of research into idiom variability based on large electronic corpora. On the theoretical level, the thesis makes the following contributions to the study of idiom variability: (1) it provides a description of the conceptual and grammatical structure of verbal idioms; (2) it demonstrates that the variability of verbal idioms is dependent on event schemas, i.e. their specificity/schematicity; (3) it shows that the core of a verbal idiom is primarily constructional in nature.