Over the last decade, biomarkers have played an increasingly important role in Alzheimer disease (AD) drug development. Biomarkers can improve patient selection and stratification, increase diagnostic accuracy, provide important prognostic information regarding future risk of clinical or radiological disease progression, and assist in monitoring disease severity or response to treatment. Through these roles, biomarkers can enrich clinical trial populations with study cohorts that have the target pathology and facilitate drug advancement using potentially smaller cohorts and shorter durations of trials. In this chapter, we review the main roles of biomarkers in AD drug development, explain the context of use for different biomarkers, and provide examples of successful biomarker utilization in AD clinical trials. Further, we discuss the different stages of biomarker development and present future directions for how biomarkers can be used to increase the chances of success of clinical trials of emerging AD therapies.