<p>Experts and their opinions can play a pivotal role in research for decision-making, particularly where empirical data are limited or uncertain. However, the methods by which expert input is solicited, synthesised, and applied in research often lack consistency and transparency. The absence of cross-disciplinary guidelines for how expert opinion should be gathered, integrated into research, and utilised in decision-making leaves the process vulnerable to conflicts of interest, interpersonal dynamics, and researcher discretion. Using health technology assessment (HTA) as a case study, we conducted a systematic review of empirical studies examining the use of expert opinion in research for decision-making. Twenty-three studies were included, from which six broad categories of expert consultation methods were identified, ranging from informal individual consultation to structured expert elicitation protocols. Considerable variation was observed in both the methodological rigour and transparency with which expert consultation was conducted and reported. Drawing on the review findings, we propose a preliminary conceptual framework (INTEGRITY) that synthesises psychosocial, methodological and reporting factors to promote transparency, rigour, inclusivity and objectivity when incorporating expert consultation into research. For low-stakes exploratory judgements, informal consultation may be sufficient. For high-stakes policy or resource-allocation decisions that require quantitative estimates, structured and transparent approaches such as formal elicitation protocols should be considered to support inclusive, objective, rigorous, and transparent use of expert judgement in research.</p>