This chapter tackles knowledge sharing by focusing on security and trust issues. Although trust is recognized as important in security issues, few studies on information systems (ISs) deal with both trust and security. Knowledge sharing relies on sense-giving and sense-reading processes which require, encourage, and even create trust within individuals. We argue that individuals are processors of information and interpret information to create their own tacit knowledge.Recent security reports from organizations have presented that the majority of ISs security threats involve employees within the organizations. Individuals, as well as computers, maybe attacked through social engineering techniques in order to gain their trust. Despite this evidence, most of the work has focused on the control of outsider security threats rather than of insider security threats, particularly when humans are perpetrators.We propose to study insider threats through a trust factor during the knowledge sharing process. Knowledge sharers may induce insider threats for security due to trust-related attitudes and behaviours. The proposition is twofold with interviews and self-report questionnaires to collect information about the trust, and ontologies to categorize such information. The proposition is then discussed, notably in terms of problems and answers leading to study trust in security when sharing knowledge.