The purpose of this study is to review the status of differential item functioning (DIF) research in language testing, particularly as it relates to the investigation of sources (or causes) of DIF, which is a defining characteristic of the third generation DIF. This review included 110 DIF studies of language tests dated from 1985 to 2019. We found that DIF researchers did not address sources of DIF more frequently in recent years than in earlier years. Nevertheless, DIF research in language testing has expanded with new DIF analysis procedures, more grouping variables, and more diversified methods for investigating sources of DIF. In addition, in the early years of DIF research, methods to identify sources of DIF relied heavily on content analysis. This review showed that while more sophisticated statistical procedures have been adopted in recent years to address sources of DIF, understanding sources of DIF still remains a challenging task. We also discuss the pros and cons of existing methods to detect sources of DIF and implications for future investigations.