BACKGROUND AND OBJECTIVES: Peer review is ubiquitous in evaluating scientific research. While peer review of manuscripts submitted to journals has been widely studied, there has been relatively less attention paid to peer review of grant applications (despite how crucial peer review is to researchers having the means and capacity to conduct research). There is spirited debate in academic community forums (including on social media) about the perceived benefits and limitations of grant peer review. The aim of our study was to understand the experiences and challenges faced by grant peer reviewers. METHODS: Therefore, we conducted qualitative interviews with 18 members of grant review panels-the Chairs, peer reviewers and Scientific Officers of a national funding agency-that highlight threats to the integrity of grant peer review. RESULTS: We identified three threats: (1) lack of training and limited opportunities to learn, (2) challenges in differentiating and rating applications of similar strength, and (3) reviewers weighting reputations and relationships in the review process to differentiate grant applications of a similar strength. These threats were compounded by reviewers' stretched resources or lack of time. Our data also highlighted the essential role of the Chair in ensuring transparency and rigorous grant peer review. CONCLUSIONS: As researchers continue to evaluate the threats to grant peer review, the reality of stretched resources and time must be considered. We call on funders and academic institutions to implement practices that reduce reviewer burden.