This paper explores the phenomenon of "informal know-how trading" between rival and non-rival firms, particularly in the US steel minimill industry. The author, Eric von Hippel, observes that engineers within these firms form informal networks to exchange proprietary technical knowledge, which he terms "know-how." This behavior is seen as a form of cooperative R&D, where firms benefit from sharing and learning from each other's innovations. Von Hippel argues that this form of trading can be economically rational under certain conditions. He uses a "Prisoner's Dilemma" framework to analyze the economic incentives involved. When the value of proprietary know-how is relatively low, firms may find it beneficial to trade it, as the potential gains from cooperation outweigh the risks of defecting. However, when the value of know-how is high, firms may choose to keep it secret to maintain a competitive advantage. The paper also discusses the broader implications of informal know-how trading. It suggests that this form of cooperation may be more effective than formal agreements to perform R&D or license proprietary knowledge due to lower transaction costs and greater flexibility. Additionally, von Hippel notes that informal know-how trading may be applicable to other industries and contexts where proprietary know-how is a significant competitive advantage. The study includes a case study of the US steel minimill industry, where von Hippel found extensive trading of proprietary process know-how among firms. He also examines other empirical data from various industries, suggesting that informal know-how trading is a widespread phenomenon. Finally, von Hippel discusses the potential for more complex trading strategies and the role of individuals within trading networks. He concludes that further research is needed to fully understand the generality and implications of informal know-how trading.This paper explores the phenomenon of "informal know-how trading" between rival and non-rival firms, particularly in the US steel minimill industry. The author, Eric von Hippel, observes that engineers within these firms form informal networks to exchange proprietary technical knowledge, which he terms "know-how." This behavior is seen as a form of cooperative R&D, where firms benefit from sharing and learning from each other's innovations. Von Hippel argues that this form of trading can be economically rational under certain conditions. He uses a "Prisoner's Dilemma" framework to analyze the economic incentives involved. When the value of proprietary know-how is relatively low, firms may find it beneficial to trade it, as the potential gains from cooperation outweigh the risks of defecting. However, when the value of know-how is high, firms may choose to keep it secret to maintain a competitive advantage. The paper also discusses the broader implications of informal know-how trading. It suggests that this form of cooperation may be more effective than formal agreements to perform R&D or license proprietary knowledge due to lower transaction costs and greater flexibility. Additionally, von Hippel notes that informal know-how trading may be applicable to other industries and contexts where proprietary know-how is a significant competitive advantage. The study includes a case study of the US steel minimill industry, where von Hippel found extensive trading of proprietary process know-how among firms. He also examines other empirical data from various industries, suggesting that informal know-how trading is a widespread phenomenon. Finally, von Hippel discusses the potential for more complex trading strategies and the role of individuals within trading networks. He concludes that further research is needed to fully understand the generality and implications of informal know-how trading.