This paper reviews the key design features, accomplishments of and lessons learned from two regional group procurement mechanisms dealing with vaccines that have been in operation for more than 25 years. The Pan American Health Organization (PAHO) EPI Revolving Fund purchases vaccines and immunization supplies on behalf of more than 35 countries in the Latin American and Caribbean region. Based on a 'central contracting' model, the program handles most aspects of procurement-from tendering to contracting with and paying producers--using a common fund to pay producers before being reimbursed by countries once goods are received in-country. The Gulf Cooperation Council (GCC) Group Purchasing Program among seven Persian Gulf States issues joint tenders for vaccines, as well as drugs and other medical goods. Through this 'group contracting' program, countries are responsible for contracting with and paying producers on their own, once the group has selected winning bids. Both programs have experienced substantial growth in the past two decades and are considered to have contributed to or accelerated achievements of immunization programs in both regions, including the introduction of new vaccines. The paper identifies several features of both programs--both those designed to attract country participation and those designed to ensure the programs' financial viability--which help explain their success and longevity.