This article examines the rise of nanotechnology-specific codes of conduct (nano-codes) as a private governance mechanism to manage potential risks and promote the technology. It examines their effectiveness as well as their legitimacy as regulatory instruments in the public domain. The study first maps the rise of voluntary nano-codes and the roles played by different actors. Focusing on five specific nano-codes, the article then discusses their adequacy in terms of scientific uncertainty, gaps in existing regulatory regimes, and broader societal concerns. It concludes that these voluntary nano-codes have weaknesses including a lack of explicit standards on which to base independent monitoring, as well as no sanctions for poor compliance. At the same time it also highlights the potential power of these governance mechanisms under conditions of uncertainty and co-regulation with government. It is likely that nano-codes will become the “first cut” of a new governance regime for nanotechnologies.