Multilingual teams in which people speaking different native languages work together are increasingly common in modern organizations. Previous research suggests that multilingual teams have the potential to incorporate information and human resource at a global scale. This potential, however, is hard to fulfill when language related issues, such as a lack of fluency in a team’s common language or clustering in native language based subgroups, impede team members from communicating efficiently. To date, there is little empirical knowledge of how members of multilingual teams handle these communication issues through their daily practice of language use. In this dissertation, I examined the dynamics of language use and its effects on daily communication in multilingual teams through an in-depth field study guided by grounded theory. Six multilingual teams participated in this study. All these teams used English as their only common language at work. I conducted over 100 hours of observation with each team and collected three types of data regarding people’s language use during daily communication events: text-based observation notes that documented the contextual information of each communication event and people’s language use behavior during the event; audio-recorded retrospection that reported people's interpretations of everybody’s language use behavior during the event; and images donated by participants that captured how people used various tools and materials to facilitate communication when needed. Through multiple iterations of coding, I identified several patterns of language use that tell 1) when and why members of multilingual teams used English rather than their native language, and vice versa, at work, and 2) how they incorporated various strategies, if any, into the practice of using either English or their native language such that they could maximize the values and minimize the costs of their language choice. Based on these findings, I discuss how the current research added to previous knowledge on multilingual teamwork, language choice and conversational grounding. Future research directions and design implications deriving from these findings are also outlined at the end of this dissertation.