A Practical Guide to Using Qualitative Research with Randomized Controlled Trials focuses on qualitative research, emphasizing subjectivity, flexibility, open data collection, depth, and context, and randomized controlled trials (RCTs), emphasizing objectivity, standardization, measurement, and a key goal of bias reduction. The differences between the two methodologies make their combination an interesting ‘extreme case’ of mixed methods research. The book is about how to use qualitative research when preparing for, undertaking, or implementing the results of an RCT. The focus is on a range of designs, including pilot and pragmatic RCTs. The book focuses on health research because the majority of RCTs are undertaken in this field. It is written for researchers who are undertaking or planning to undertake qualitative research with RCTs. The book may also be useful to researchers leading RCTs and graduate students. It does not explain how to do qualitative research or how to do RCTs. The book focuses on how to undertake qualitative research in the specific context of RCTs. The book is divided into three parts. Part 2 offers the practical guidance promised within the title of the book. Each chapter focuses on key steps when undertaking qualitative research in the context of RCTs: writing a proposal, selecting research questions, collecting data, analysing data, integrating qualitative and quantitative components, and publishing. Part 1 sets the scene for the practical aspects. Part 3 focuses on the human beings we work closely with during research and offers practical advice to ensure these interactions are positive and fruitful.