Temporary employment is estimated to be around 20% of the US auto industry. Based on extensive fieldwork at the big three Japanese factories (Toyota, Honda and Nissan) and auto suppliers, this paper documents the impact of temporary workers on the shop floor and their role in the context of lean production. This article fills an important gap in the existing research as one of the few empirical studies that examine the effects of temp work in the auto industry. We conclude that the just-in-time workforce as a human buffer for sick or injured team members and an addition when production volume goes up is a core structural aspect of lean production. The lean, loyal and long-term principles of mature lean production to protect permanent workers are based on the disloyal and short-term use of temporary workers.