Patient-reported outcome (PRO) measures must provide evidence that their development followed a rigorous process for ensuring their content validity. To this end, the collection of data is performed through qualitative interviews that allow for the elicitation of in-depth spontaneous reports of the patients' experiences with their condition and/or its treatment. This paper provides a review of qualitative research applied to PRO measure development. A clear definition of what is a qualitative research interview is given as well as information about the form and content of qualitative interviews required for developing PRO measures. Particular attention is paid to the description of interviewing approaches (e.g., semi-structured and in-depth interviews, individual vs. focus group interviews). Information about how to get prepared for a qualitative interview is provided with the description of how to develop discussion guides for exploratory or cognitive interviews. Interviewing patients to obtain knowledge regarding their illness experience requires interpersonal and communication skills to facilitate patients' expression. Those skills are described in details, as well as the skills needed to facilitate focus groups and to interview children, adolescents and the elderly. Special attention is also given to quality assurance and interview training. The paper ends on ethical considerations since interviewing for the development of PROs is performed in a context of illness and vulnerability. Therefore, it is all the more important that, in addition to soliciting informed consent, respectful interactions be ensured throughout the interview process.