Accumulating evidence suggests that young children spend excessive time being sedentary. The purpose of this systematic review was to determine the relationship between sedentary behaviours and health indicators during the early years (ages 0-4 years). Using the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) framework, this review aimed to present the best available evidence on the threshold of sedentary behaviour associated with healthy measures of adiposity, bone health, motor skill development, psychosocial health, cognitive development, and cardiometabolic health indicators in infants, toddlers, and preschoolers. Online databases, personal libraries, and government documents were searched for relevant studies. Studies that included an intervention (or experimental) group or prospective analysis were included. Twenty-one unique studies, representing 23 papers and 22 417 participants, met inclusion criteria; 7 studies included information on infants, 13 on toddlers, and 10 on preschoolers. Of these, 11, 6, and 8 studies reported data on adiposity, psychosocial health, and cognitive development, respectively. No included study reported on motor skill development, bone, or cardiometabolic health indicators. In conclusion, this review found low- to moderate-quality evidence to suggest that increased television viewing is associated with unfavourable measures of adiposity and decreased scores on measures of psychosocial health and cognitive development. No evidence existed to indicate that television viewing is beneficial for improving psychosocial health or cognitive development. In several instances a dose-response relationship was evident between increased time spent watching television and decreased psychosocial health or cognitive development. This work may be used as evidence to inform public health guidelines. (PROSPERO registration: CRD4011001280.).