Randomized clinical trials (RCTs) are often referred to as the "gold standard" of clinical research. However, in its current state, the U.S. clinical trials enterprise faces substantial challenges to the efficient and effective conduct of research. Streamlined approaches to RCTs, such as large simple trials (LSTs), may provide opportunities for progress on these challenges. Clinical trials support the development of new medical products and the evaluation of existing products by generating knowledge about safety and efficacy in pre- and post-marketing settings and serve to inform medical decision making and medical product development. Although well-designed and -implemented clinical trials can provide robust evidence, a gap exists between the evidence needs of a continuously learning health system, in which all medical decisions are based on the best available evidence, and the reality, in which the generation of timely and practical evidence faces significant barriers. Large Simple Trials and Knowledge Generation in a Learning Health System is the summary of a workshop convened by the Institute of Medicine's Roundtable on Value & Science-Driven Health Care and the Forum on Drug Discovery, Development, and Translation. Experts from a wide range of disciplines-including health information technology, research funding, clinical research methods, statistics, patients, product development, medical product regulation, and clinical outcomes research-met to marshal a better understanding of the issues, options, and approaches to accelerating the use of LSTs. This publication summarizes discussions on the potential of LSTs to improve the speed and practicality of knowledge generation for medical decision making and medical product development, including efficacy and effectiveness assessments, in a continuously learning health system. Large Simple Trials and Knowledge Generation in a Learning Health System explores acceleration of the use of LSTs to improve the speed and practicality of knowledge generation for medical decision making and medical product development; considers the concepts of LST design, examples of successful LSTs, the relative advantages of LSTs, and the infrastructure needed to build LST capacity as a routine function of care; identifies structural, cultural, and regulatory barriers hindering the development of an enhanced LST capacity; discusses needs and strategies in building public demand for and participation in LSTs; and considers near-term strategies for accelerating progress in the uptake of LSTs in the United States.