In conducting clinical trials, we evaluate the most promising findings from translational research, gain perspective about mechanisms of action, and strive to identify treatments that are both effective and safe for a wide range of Alzheimer’s disease (AD) patients. To assess the effectiveness of any clinical trial drug, we must first identify, screen, and follow representative participants through multiple trial phases, a years-long duration between discovery and efficacy. Clinical trials depend upon large, diverse, and well-characterized participant samples. Clinical trial populations must include representative variability across sociodemographic characteristics to capture unequal risk and potentially varied responses to treatment. Diverse participant samples are our best tool for generalizing effects to patient populations but are also one of our largest barriers to the timely and complete investigation of new treatments. Recruitment and retention receive some of the blame for the long, protracted timeline of clinical trials. Finding solutions for recruitment challenges will therefore improve the overall efficiency of study trials and the speed of drug discovery.