This title illustrates the ways in which linguistic variation can be explored through corpus-based investigation. Two major kinds of research questions are considered: variation in the use of a particular linguistic feature, and variation across dialects or registers. Part 1: "Exploring variation in the use of linguistic features" focuses on the study of specific words, expressions, or grammatical constructions, to study variation in the use of a particular linguistic feature. Part 2: "Exploring dialect and register variation" describes salient characteristics of dialects or registers and the patterns of variation across varieties. Part 3: "Exploring Historical Variation" applies these same two major perspectives to historical variation. One recurring theme is the extent to which linguistic variation depends on register differences, reflecting the importance of register as a key methodological and thematic concern in current corpus linguistic research.