In this chapter, we empirically investigate an important question of "how does knowledge sharing and knowledge leakage impact the alliance performance in dyadic coopetitive alliance settings that involve small and medium sized enterprises (SMEs)." Taking the perspective of the focal SME to address this question, we posit that while knowledge sharing positively associates with alliance performance, inadvertent knowledge leakage is negatively related to performance. We further postulate that under the conditions of high knowledge leakage, the positive impacts of knowledge sharing on performance would be reduced. Our structural model results based on a survey of 186 SMEs in the high-tech and knowledge-intensive industries in Sweden show support for two of the hypothesized relation- ships. More specically, the results show that knowledge sharing has a positive effect on alliance performance but knowledge leakage has an insignificant direct effect on performance. However, knowledge leakage plays a negative moderating role on the relationship between knowledge sharing and performance. We contribute by demonstrating the effects of knowledge sharing and leakage in under-researched but important dyadic one-to-one coopetitive alliances involving SMEs.