OBJECTIVE - To describe the design, measurements and fieldwork of the IDEFICS (Identification and prevention of dietary- and lifestyle-induced health effects in children and infants) physical activity and body composition validation study, and to determine the potential and limitations of the data obtained. DESIGN - Multicentre validation study. SUBJECTS - A total of 98 children from four different European countries (age: 4-10 years). METHODS - An 8-day measurement protocol was carried out in all children using a collaborative protocol. Reference methods were the doubly labelled water method for physical activity, and a three- and a four-compartment model for body composition. Investigated field methods were accelerometers, a physical activity questionnaire and various anthropometric measurements. RESULTS - For the validation of physical activity field methods, it was possible to gather data from 83 to 89 children, laying the basis for age- and sex-specific results. The validation of body composition field methods is possible in 64-80 children and allows sex-specific analyses but has only limited statistical power in the youngest age group (<6 years). The amount of activity energy expenditure (AEE) varied between centres, sexes and age groups, with boys and older children having higher estimates of AEE. After normalisation of AEE by body weight, most group-specific differences diminished, except for country-specific differences. CONCLUSION - The IDEFICS validation study will allow age- and sex-specific investigation of questions pertaining to the validity of several field methods of body composition and physical activity, using established reference methods in four different European countries. From the participant analyses it can be concluded that the compliance for the investigated field methods was higher than that for the reference methods used in this validation study.