Soybean ('Glycine max') is a tropical legume that can be grown on many soil types under a wide range of climatic conditions. It is a native of eastern Asia and originally grew wild in China, Manchuria, Korea and Japan. This manual outlines how to grow soybean successfully in Cambodia under rainfed upland conditions. Soybean production in Cambodia has been growing steadily since 1980 (Figure 1) and exceeded 100,000 tonnes per year in 2005. The main soybean production area for the past 30 years has been the province of Kampong Cham. However, in recent years, soybean production has increased in north-western Cambodia, especially in the province of Battambang. Soybean is also grown in other provinces, including Siem Reap, Kandal and Takeo. Soybean is usually grown in the main wet season, and the crop fits well in an upland rotation in combination with maize, sesame and peanut. The roots of legume crops such as soybean have nodules formed by rhizobium bacteria ('Bradyrhizobium' spp.), which obtain nitrogen from the atmosphere; this nitrogen becomes available to both the soybean and following crops. Rhizobia are not always present in the soil and may need to be added to the seed at sowing time (see Section 8).