New technology, fueled by the Internet’s commercialization, has led to new types of business partner connectivity. However, statistics show business-to-business (B2B) electronic commerce (EC) and electronic market growth are slower than anticipated (Bartlett, 2001). Realizing the full potential of this emerging inter-organizational connectivity requires understanding what facilitates its adoption and diffusion. Given the need for research in this area, this chapter reviews the existing inter-organizational information systems (IOISs) adoption and diffusion research. The chapter analyzes twenty-five empirical inter-organizational information system adoption and diffusion studies. This study identifies several IOIS types and lists and categorizes variables found to significantly influence adoption and diffusion of each IOIS type. This study has two main contributions. The study finds that variables found to significantly influence IOIS adoption and diffusion fall into three categories: inter-organizational, organizational and technical. The study also brings the IOIS adoption and diffusion literature together. As such, this study provides a starting point for conducting research on emerging IOISs.