To provide a user's guide for selecting an appropriate method to assess sedentary behaviours among children and adolescents. While recommendations regarding specific instruments are not provided, the guide offers information about key attributes and considerations for objective (accelerometry; inclinometers; direct observation; screen monitoring devices) and subjective (self-report; parent report; and time use diaries/logs) approaches to assess sedentary behaviour Attributes of instruments and other factors to be considered in the selection of assessment instruments include: population (age); sample size; respondent burden; method/delivery mode; assessment time frame; physical activity information required (data output); data management; measurement error; cost (instrument and administration) and other limitations. Expert consensus among members of the Australasian Child and Adolescent Obesity Research Network's (ACAORN) Physical Activity and Sedentary Behaviour Special Interest Group. We developed decision flow charts to assist researchers and practitioners select an appropriate method of assessing sedentary behaviour, identified attributes of each method and described five real-life scenarios to illustrate considerations associated with the selection of each method of measurement. It is important that researchers, practitioners and policy makers understand the strengths and limitations of different methods of assessing sedentary behaviour among youth, and are guided on selection of the most appropriate instrument/s to suit their needs.