Purpose -The purpose of this paper is to examine security incident response practices of information technology (IT) security practitioners as a diagnostic work process, including the preparation phase, detection, and analysis of anomalies. Design/methodology/approach -The data set consisted of 16 semi-structured interviews with IT security practitioners from seven organizational types (e.g. academic, government, and private). The interviews were analyzed using qualitative description with constant comparison and inductive analysis of the data to analyze diagnostic work during security incident response. Findings -The analysis shows that security incident response is a highly collaborative activity, which may involve practitioners developing their own tools to perform specific tasks. The results also show that diagnosis during incident response is complicated by practitioners' need to rely on tacit knowledge, as well as usability issues with security tools. Research limitations/implications -Owing to the nature of semi-structured interviews, not all participants discussed security incident response at the same level of detail. More data are required to generalize and refine the findings. Originality/value -The contribution of the work is twofold. First, using empirical data, the paper analyzes and describes the tasks, skills, strategies, and tools that security practitioners use to diagnose security incidents. The findings enhance the research community's understanding of the diagnostic work during security incident response. Second, the paper identifies opportunities for future research directions related to improving security tools.