Intelligent systems use applied artificial intelligence techniques with the aim of reaching, in a specific domain, a level of analysis and performance comparable to human experts. Intelligent systems are able to engage in complex inference processes, necessary for evaluating alternative options, presenting high quality conclusions and advice, and to offer explanations about the rationale that led to such conclusions. Knowledge Management is a key process associated to the development of intelligent systems, since it elicits, codifies, validates and relates the knowledge elements that are stored in the system’s Knowledge Base. The paper focus on the issues involved in the design of the THEMIS intelligent system’s Knowledge Base, and of the cooperative and participatory processes applied for knowledge elicitation, referring the usage of ontologies and UML use-cases. THEMIS project purpose is to develop an intelligent system to support complex and stressful Emergency Management activities. The outcome of the described Knowledge Management process can be determinant to quality of the user experience when exploiting the system, since most of the pragmatic and hedonic qualities of the interactions of an intelligent system are closely related with the characteristics of the underlying knowledge base and inferencing process.