Focus groups are a means of gathering qualitative data from a group of participants who discuss a given topic. This method has been used in health care research for the past 30 years, but has seen limited use in radiation therapy research. Focus group discussions are a useful tool for investigating a variety of educational, training and clinical issues from the perspective of practitioners, students and patients. This paper reviews the issues associated with using focus groups as a means of data collection. In particular, it addresses some of the decisions which have to be made about group composition and conduct of the discussions. The literature review is contextualised using a recent example of how the authors used focus groups to investigate fitness to practise in radiation therapy. Other challenges such as familiarity between participants and researchers, power relationships and anonymity are addressed. The paper concludes with a consideration of data analysis.