Modern rice varieties (MVs) were introduced in tropical Asia in 1965 with the widespread testing of Taichung Native 1 and release of IR8. China had earlier developed fertilizer-responsive semidwarf varieties using a gene for dwarfing that later went into the parentage of IR8 and other semidwarf varieties. By 1980 nearly 40% of the rice area in South and Southeast Asia was planted to MVs and new hybrid rices were being grown in China. Despite their rapid spread, many questions are still asked about the spread of MVs and the technology associated with them. Where are MVs grown? What farmers grow them? What contribution have they made to production? This publication brings together the information that is available to answer these questions. The first section documents the development and introduction of MVs in major South and Southeast Asian rice growing countries. The second section describes the methodology used to estimate the contribution of improved varieties to the increases in production that have occurred since 1965. It is estimated that by the early 1980s MVs contributed about $4.5 billion annually to the value of rice produced in Asia. In the third section of the monograph, the authors review many earlier studies that describe the MV adoption pattern and associated factors. Special attention is given to the question of farm size and adoption. It is hoped that this compilation will answer many of the questions that are raised about MVs. IRRI recognizes that technological innovations alone cannot solve the pressing problems of development in the Third World, but at the same time we believe that new technology is one of the necessary components for a solution. We are proud to be partners with Asia's many national rice research and extension program that have made possible the development and spread of modem rice varieties.