The estimation of yield loss caused by diseases requires techniques for a) quantification of the pathogen or disease, b) generation of epidemics of different durations and intensities, c) experiments to provide data for characterizing the disease-loss relationship, d) modeling and e) field surveys. Techniques for (a)-(c) are relatively well-developed for tropical rice, in comparison to techniques for (d) and (e). Disease assessment methods commonly use numerical scales such as 1 (healthy) - 9 (dead) or percentage points. Yield loss experiments have been conducted in greenhouses and the field, using single hills and plots. Regression models have been developed for only a few tropical rice diseases. At IRRI, methods are being investigated to determine yield losses caused by blast, rice tungro virus, bacterial blight and sheath blight. Methodology is also being tested for conducting surveys of multiple pest infestations in farmers' fields. The paper will also describe the methodology used in selected national programs for yield loss assessment.