Vaccine shortages can result from higher-than-expected demand, interruptions in production/supply, or a lack of resources to purchase vaccines. Each of these factors has played a role in vaccine shortages in the United States during the past 20 years. Since 2000, the United States has experienced an unprecedented series of shortages of vaccines recommended for widespread use against 9 diseases, after more than 15 years without vaccine supply problems. In developing countries, the major cause of vaccine shortages is lack of resources to purchase them. Although there are several steps that could reduce the likelihood of future vaccine shortages, many would take several years to implement. Consequently, we will probably continue to see occasional shortages of vaccines in the United States in the next few years.