The purpose of this study was to investigate the extent to which overweight and obesity are challenges among primary school children in Kinondoni and Njombe districts. The study sought to investigate those aspects in terms of prevalence, causes and impacts on social, health as well as children learning behaviours and outcomes. Systematic random sampling was used to select schools while stratified sampling and simple random sampling were used in selecting pupils and teachers. Measurement of weights and height were done to determine Body Mass Index (BMI), measurements of skinfolds were also done to determine body fat percentage. Questionnaires, semi-structured interview schedule and focus group discussion guides were also used. Findings revealed an average of 13.5% children, were overweight and obese. Economy status, household occupations, nutrition and inactivity were significant causes of overweight and obesity. Hypertension, excessive sweating, teasing and peer rejection were common to obese children. In addition, overweight and obese children were reported to underperform in academic and physical activities. The study revealed that overweight and obesity were not friendly healthy conditions to children, thus a need to work it out. The study suggests for establishment of education programs through mass Medias, to raise people's awareness on implications of obesity in children's health, social, and learning behaviours and outcomes. Seven appendixes are included: (1) Pupils' Questionnaires; (2) Pupils' Focus Group Discussion Guide; (3) Teachers' Interviews; (4) Number of Children and their Weight Status in both Rural and Urban Settings (BMI Results); (5) Percentage of Children According to their Weight Status and Performance Grades in the Classroom; (6) Factors Causing Overweight and Obesity among School Children and their Level of Significance; and (7) A Map of Kinondoni and Njombe Showing the Surveyed Schools.