Zhiguo Xie's research centers on exploring what natural language utterances mean and how their meanings are derived from the structural representations. Within this broad research program, he is particularly interested in the questions of where and how human languages differ from each other. The data he uses to tackle these questions primarily come from Mandarin Chinese and English. He is also interested in bilingualism and sociolinguistics. His most recent research deals with three topics. The first one explores why certain temporal adverbs affect the grammaticality or interpretation of modal sentences. The second one studies how the notion "gradability" relates to and/or explains certain language phenomena. The third topic delves into the interaction of questions and focus expressions.