Knowledge leakage refers to loss of technological knowledge intended to stay within a firm׳s boundaries and may cause a “weakened state” in which a focal firm loses its competitive advantage and industry position. Based on multiple case studies of knowledge leakage in joint research and development (R&D) projects in large firms in Sweden, this paper makes two contributions. First, in contrast to the uni-dimensional dyadic leakage process assumed in the literature, we find that the knowledge leakage process is multi-dimensional and exists in three varieties: i) a process whereby an external party assimilates knowledge from a focal firm, ii) a process whereby an external party assimilates knowledge from another external party, and iii) a process whereby the focal firm uses knowledge already shared with an external party in such a way that it becomes sensitive. Second, where the prior literature suggests that core knowledge must be protected from leakage, we find that some core knowledge can leak without negative effects, whereas some knowledge, being non-core to a focal firm, can have severe negative effects. These insights provide novel theoretical implications and new insights into how firms can manage knowledge leakage in practice.