Dialect comparison studies have been conducted from five perspectives: internal comparison of a dialect; comparison among different dialects; comparison between a dialect and a common language used between two ethnic groups; comparison of the current situation and the historical development of a dialect; and the comparison between a dialect and classical Chinese. Internal comparison of a dialect includes comparison between old and new varieties, dialectal usage by different classes or in different occasions, comparison of the characters’ pronunciations within the dialect’s sound system, and individual dialect differences. The comparisons from the first three perspectives are synchronic, while the last two are diachronic. The current chapter focuses on the research methods and the questions that require attention, the relationship between the dialect comparison and dialect study, as well as the significance of the dialect history study, etc.