Background: Over the past few decades, traditional foods have been displaced by ultraprocessed foods (UPFs), with the latter being associated with various health problems. The consumption of UPFs plays an important role in the development of childhood obesity worldwide. The aim of this thesis was to review all existing knowledge regarding the association between the consumption of UPFs with obesity and cardiometabolic risk factors among children and adolescents. Furthermore, using data from children and adolescents ≥2-18 years enrolled in the Hellenic National Nutrition and Health Survey (HNNHS), to: a) assess the main food groups contributing to UPF consumption, b) assess the association of UPF consumption with weight status, c) to estimate the nutrient content contributed by UPFs and d) to examine associations between consumption of UPFs and nutrient intake recommended in international guidelines for the prevention of NCDs Methods: Firstly, a systematic search on the impact of ultra-processed foods on obesity and cardiometabolic co-morbidities in children and adolescents conducted using the Preferred Reporting Items for Systematic Review (PRISMA-P) criteria included observational studies up to February 2022. Based on the inclusion and exclusion criteria, seventeen observational studies involving children and adolescents aged ≤18 years were included. Of these, 9 were cross-sectional studies, 7 were cohort studies, and one study combined the results of both cross-sectional and 3 cohort studies. In particular, 14 studies investigated the relationship between UPFs intake and overweight/obesity, while 8 examined the relationship between UPFs and cardiometabolic risk factors. Subsequently, data from 478 children and adolescents ≥2-18 years enrolled in the Hellenic National Nutrition and Health Survey (HNNHS) - a cross-sectional study, were analyzed. Two 24hr recalls were used to estimate dietary intake. Following this, the UPFs were classified based on the NOVA system. The proportion of UPFs’ contribution to the daily energy intake was calculated. Main UPF food contributors were derived for the total population, by weight status and across tertiles. The association between weight status and UPF intake for the main contributors was examined using generalized linear models. The prevalence of inadequate or excessive intakes of macro- and micronutrients were defined using the dietary recommendations of the American Heart Association for macronutrients and the Estimated Average Requirement (EAR) for micronutrients. A linear regression model adjusted for covariates was used to test trends in the dietary energy share provided by UPFs across tertiles. Results: Results from systematic review demonstrated that most studies (14/17) found that increased UPF consumption was associated with a higher prevalence of overweight/obesity and cardiometabolic risk factors such as dyslipidemia, blood pressure, and metabolic syndrome in children and adolescents, while 4/17 (3 cross-sectional studies and 1 cohort study) did not show any correlation. According to study quality assessment tools, the majority of cohort and crosssectional studies were classified as good quality based on NIH (National Institutes of Health) and NewCastle-Ottawa, respectively. Findings of the original research showed that the percentage of total daily energy provided in the diets of children and adolescents by UPFs was 41%. Four major food groups were found to contribute > 10% of total UPF intake: ready-to-eat/heat dishes (36.2%), sweet grain products (21.4%), savoury snacks (15.4%) and sweets (12.9%). These provided 86% of the total UPF intake, with no significant differences between children's weight status. Total UPF intake was not associated with body weight status, but the likelihood of obesity significantly increased for children consuming savoury snacks and baked goods of > 62% of their total caloric intake. Regarding diet quality children with higher UPFs consumption consumed more energy [1416kcal (95% CI 1128, 1776) 1st tertile vs. 1920kcal (95% CI 1477, 2426) 3rd tertile], carbohydrates (from 42.6 to 43.7% kcal), added sugars (from 4.5 to 8.1% kcal), saturated fats (from 14.9 to 16% kcal), polyunsaturated fats (from 4.9 to 5.2% kcal) and sodium (from 1641 to 2485 mg/day) compared to children in the lowest tertile of UPF intake. In contrast, UPF intake was negatively associated with the consumption of protein [15.9% (14, 19.4) 1st tertile vs 14.6% (11.7, 16.2) 3rd tertile], and potassium [(<70% AI), (1953 (1953, 2576) 1st tertile vs 1799 (1398, 2307) mg/day 3rd tertile)]. The prevalence of excessive intake of added sugars ≥10% of total energy (17.2% 1st tertile vs 39.7% 3rd tertile) increased significantly across tertiles of UPFs. Conclusions: There is a positive relationship between UPF intake and overweight/obesity, as well as cardiometabolic co-morbidities in children and adolescents according to our systematic review. Regarding the results of the Hellenic National Nutrition and Health Survey (HNNHS), the contribution of UPFs to total energy intake was high for all children irrespective of weight status. It was also highlighted that a greater dietary share of UPFs demonstrates a poor quality of children’s and adolescents’ diets, which are characterized by higher dietary content of NCD-promoting and 4 lower dietary content of NCD-protecting nutrients. This evidence adds to previous studies indicating that more effective public health strategies to improve diet quality are needed, aiming to promote the reduction of the obesity pandemic among children.