Any issues in the recruitment and retention of subjects for a clinical trial can result in delays and unnecessary costs, and may even potentially undermine its results. Each clinical phase faces its own specific recruitment challenges that need to be overcome: however, special populations and patient cohorts are increasingly included in early-phase studies to ensure the success of a therapy. Dr Katrien Lemmens of SGS discusses how, irrespective of the clinical phase, the success of a trial relies heavily on the enrolment of subjects, and this may be made more challenging depending on the specific needs of the study design, the trial phase and the population’s characteristics.