International guidelines support exercise as beneficial to a range of health-related outcomes for people with cancer. Exercise oncology trials report few adverse events (AE), but the extent to which these harms have been assessed and reported is unclear. As the field moves towards exploring the effect of higher intensity exercise, understudied cancer populations and more unsupervised methods of delivery, there is a clear need to concurrently better understand the potential harms of exercise. The overall aim of this thesis was to investigate and improve harms assessment and reporting in exercise oncology trials, which was achieved through three research questions. The first was to summarise how AEs are assessed and reported in exercise oncology. To address this, a review of exercise interventions that support international, clinical exercise guidelines for people with cancer was conducted. These findings indicate that exercise harms have been underexplored and poorly reported in exercise oncology, owing to half of trials not reporting AEs, and even fewer providing a clear method for their assessment. The second was to describe the current standard of harms reporting in oncology and exercise trials via a narrative review. These findings indicate that governing regulation for harms reporting in clinical trials has poor relevance to behavioural and lifestyle trials (i.e., exercise). Additionally, no harms reporting templates exist for exercise trials, comparative to validated harms assessment systems embedded within oncological trials. The final aim was to compare findings from aim one and two to make recommendations for an exercise oncology harms assessment protocol. Such a protocol may draw from pre-existing AE reporting protocols within the oncology and medical fields, but will need to incorporate clinically relevant and meaningful elements for clinical exercise practice for its successful uptake and use within exercise oncology trials. Improving the availability of published, transparent harms outcomes will enable exercise professionals to enhance the appropriateness of exercise prescriptions for their clients with cancer, without compromising benefit.