Objective: The California heat illness prevention study (CHIPS) devised methodology and collected physiological data to assess heat related illness (HRI) risk in Latino farmworkers. Methods: Bilingual researchers monitored HRI across a workshift, recording core temperature, work rate (metabolic equivalents [METs]), and heart rate at minute intervals. Hydration status was assessed by changes in weight and blood osmolality. Personal data loggers and a weather station measured exposure to heat. Interviewer administered questionnaires were used to collect demographic and occupational information.