Abstract Sedentary behaviour (SB) is considered a risk factor for non-communicable diseases and long periods of sedentary time (ST) have been associated with an increased risk of mortality in adult populations. Although investigation in young children (aged 1 to 5.9) is less consistent, this period has been identified as a critical period in which sedentary habits may be established and track throughout life. The aim of this Doctoral thesis was to investigate the prevalence, correlates and measurements of SB in toddlers (aged 1 to 2.9 years) and pre-schoolers (aged 3 to 5.9 years). This thesi...