The nature of teamwork in professional service firms, as in many other knowledge-intensive environments, is evolving from highly structured project teams to more fluid, open-ended, peer-to-peer collaboration such as that between powerful, high-autonomy partners. This shift is especially challenging because senior-level collaboration requires peers from different practice groups or offices with different sub-cultures to negotiate task allocation, credit recognition, and decision-making norms, which can be difficult and politically charged. Increased partner-level collaboration is further complicated by other trends in the PSF arena such as expertise specialization, heightened professional mobility, and increased competition. Yet, this phenomenon remains largely under-researched and under-theorized. The chapter therefore lays out a research agenda focusing on opportunities to better understand peer collaboration in PSFs. In addition, the chapter identifies ways that recent changes in the professional sector challenge our understanding of traditional teamwork, and it identifies specific gaps that deserve scholarly attention.