This study explores interruption patterns among software developers who program in pairs versus those who program solo. Ethnographic observations indicate that interruption length, content, type, occurrence time, and interrupter and interruptee strategies differed markedly for radically collocated pair programmers versus the programmers who primarily worked alone. After presenting an analysis of 242 interruptions drawn from more than 40 hours of observation data, we discuss how team configuration and work setting influenced how and when developers handled interruptions. We then suggest ways that CSCW systems might better support pair programming and, more broadly, provide interruption-handling support for workers in knowledge-intensive occupations. Categories and Subject Descriptors H5.3 [Information Interfaces and Presentation]: Group and Organization Interfaces – Collaborative Computing. General Terms Management; Human Factors Keywords Pair programming, collaborative work, ethnogr...