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CONTEXT The timing of introducing complementary feeding (CF) is crucial because premature or delayed CF can be associated with adverse health outcomes in childhood and adulthood. OBJECTIVE This systematic review aims to evaluate the impact of the timing of CF introduction on health, nutrition, and developmental outcomes among normal-term infants. DATA SOURCES Electronic databases and trial registries were searched, along with the reference lists of the included studies and relevant systematic reviews. DATA EXTRACTION Two investigators independently extracted data from the included studies on a standardized data-extraction form. DATA ANALYSIS Data were meta-analyzed separately for randomized controlled trials (RCTs) and observational studies on the basis of early introduction of CF (< 3 months, < 4 months, < 6 months of age) or late introduction of CF (> 6 months, > 8 months of age). Evidence was summarized according to GRADE criteria. In total, 268 documents were included in the review, of which 7 were RCTs (from 24 articles) and 217 were observational studies (from 244 articles). Evidence from RCTs did not suggest an impact of early introduction, while low-certainty evidence from observational studies suggested that early introduction of CF (< 6 months) might increase body mass index (BMI) z score and overweight/obesity. Early introduction at < 3 months might increase BMI and odds of lower respiratory tract infection (LRTI), and early introduction at < 4 months might increase height, LRTI, and systolic and diastolic blood pressure (BP). For late introduction of CF, there was a lack of evidence from RCTs, but low-certainty evidence from observational studies suggests that late introduction of CF (> 6 months) might decrease height, BMI, and systolic and diastolic BP and might increase odds of intestinal helminth infection, while late introduction of CF (> 8 months) might increase height-for-age z score. CONCLUSION Insufficient evidence does suggest increased adiposity with early introduction of CF. Hence, the current recommendation of introduction of CF should stand, though more robust studies, especially from low- and middle-income settings, are needed. SYSTEMATIC REVIEW REGISTRATION PROSPERO registration number CRD42020218517.