Registries play an important role of identifying large groups of individuals who may be eligible for clinical trials. Major advantages include scalability, ability to reach many individuals with few resources, and to connect with individuals who may not otherwise engage in research. Major challenges include generalizability, data integrity, data validity, and study drop-out. This chapter reviews the four largest online registries in the USA: the Brain Health Registry, the Alzheimer’s Prevention Registry and GeneMatch, Cleveland Clinic’s HealthyBrains, and Alzheimer’s Association TrialMatch. Participant engagement strategies and current demographics are summarized. We also discuss a newly established program to create Trial-ready Cohort for Preclinical and Prodromal AD (TRC-PAD) with the aim to accelerate clinical trial enrollment. It is important to build a large cohort, to identify enough participants eligible and interested in clinical trials. Compatibility with smartphones, and further study of engagement and retention strategies with a focus on under-represented groups are critical to achieve our ultimate goal of trial participants representing the full US population.