Sickness presenteeism refers to the practice of continuing to work during illness. Although not always damaging, working while sick can impair employees' wellbeing and professional functioning. As the causes of presenteeism are multi-faceted, insight into the experiences of different occupational groups is needed. This study examines the prevalence of sickness presenteeism among academic employees, the factors that encourage such behaviour and the implications for wellbeing and performance. Qualitative data obtained from 5209 academics working in UK universities were subjected to thematic content analysis. Most respondents (90%) reported working while sick at least sometimes, with more than half (52%) doing so often or always. Seven interlinked themes were identified: workload pressure; non-negotiable deadlines; lack of cover; culture and working conditions; being professional, working strategically and personal choice. Although presenteeism among academics can be therapeutic and functional, the findings suggest it has serious implications for their wellbeing and performance.