The U.S. Preventive Services Task Force (USPSTF) makes recommendations about clinical preventive services. The USPSTF examines chains of direct and indirect evidence to demonstrate the effectiveness of a clinical preventive service. Missing links across the chains of evidence reflect gaps in the research. Evidence gaps can occur for preventive services that receive a letter grade recommendation and those that receive an I statement (insufficient evidence). This article describes the types of evidence gaps that the USPSTF encounters across its various recommendations and how the USPSTF identifies and communicates these gaps to researchers and policymakers, who can help generate the needed evidence. Common types of evidence gaps include limited evidence in primary care settings and populations, a lack of appropriate health outcomes, limited evidence linking behavior change to health outcomes, and a lack of evidence for effective preventive services in diverse populations. The USPSTF annual report to Congress focuses on the evidence gaps of new recommendations from the past year and is sent to leading research funding agencies. The Office of Disease Prevention at NIH uses this report to help direct future funding opportunities that may address these evidence gaps. The USPSTF plays a critical role in highlighting the information needed to advance the science to optimize the use of clinical preventive services in primary care.