Recruitment of subjects in randomized controlled trials is the process of screening and enrolling a predetermined number of subjects within a planned time. Differences between projected and actual recruitment rates are common and, when large, form a serious threat to the successful completion of the trial. Recruitment can be enhanced by 1) use of different, concurrent recruitment methods; 2) avoiding underestimations of the costs and time intensity of subject recruitment; and 3) regular monitoring of the efficiency of the recruitment effort. The recruitment process can be complicated by legal requirements of proper representation of sex and race characteristics or by stringent entry criteria. Complex designs (e.g., trials with a pre-randomization placebo run-in, split-mouth designs) have not yet shown to be cost-effective and may endanger recruitment success. Enhancing both subject retention and protocol compliance requires careful planning, attention to product design and staff training. The impact on trial findings is discussed.