This study examined the uncertain responses of 56 alleged sexual abuse victims, aged 5–17 years, testifying in Scottish criminal court trials. Don't know/remember ground rules were explained to 38% of the children and each child reported uncertainty in response to 15% of the questions on average. Uncertain responding was associated with expressions of resistance and confusion, questioning context (proportionally more regarding substantive than non‐substantive issues), question content (least to disclosure‐focused questions), utterance type (more to directives, particularly those posed by defense lawyers; more to recall‐based than recognition prompts), and age (children in mid‐adolescence were less likely to respond uncertainly than those who were either older or younger). There were no associations between expressions of uncertainty and ground rule administration, or with whether or not the question focused on central rather than peripheral details about the alleged crimes. Findings highlight concerns surrounding preparatory procedures to help witnesses, especially adolescents, indicate uncertainty when testifying. Copyright © 2017 John Wiley & Sons, Ltd.